In the 1960s and ’70s, the Kesey brothers were both tastemakers in America.

Ken Kesey (pronounced KEE-see) was a siren of the psychedelic era, a celebrated novelist and counterculture luminary who wrote “One Flew Over the Cuckoo’s Nest” and popularized lysergic acid diethylamide, or LSD. (He died in 2001 at 66.)

His younger brother, Chuck Kesey, also influenced the culture — and also with acid, if a more benign variety.

In 1970, the company he founded with his wife introduced Lactobacillus acidophilus bacteria into milk, fermenting it by converting its sugar into lactic acid and producing what was billed as the first commercially sold yogurt in the country that contained probiotics, a landmark in the natural foods movement.

He died on Nov. 6 at his home in Eugene, Ore., his daughter, Sheryl Kesey Thompson, said. He was 87.

His company, now called Nancy’s Probiotic Foods — after Mr. Kesey’s old bookkeeper, Nancy Van Brasch Hamren, with whom he developed the original honey-sweetened yogurt recipe — produces and distributes nationwide around 40 cultured products, including cottage cheese and kefir, which is fermented milk made from a culture of yeasts and bacteria. (Probiotics, live microorganisms that may confer health benefits, are found in cultured dairy products as well as in fermented foods like miso, sauerkraut and kimchi.)

Nancy’s, which is privately owned, was reported in 2017 to have had sales of $27 million the previous year. That is just a tiny corner of the market it helped create: In America, probiotic dairy products now generate about $13 billion in annual revenue.

“Chuck is one of the nicest people in the world,” Tom Wolfe wrote in “The Electric Kool-Aid Acid Test” (1968), his account of the Kesey brothers’ cross-country gambol in a Day-Glo-painted bus as members of the gleefully anti-establishment Merry Pranksters.

Chuck Kesey was, Mr. Wolfe added, “a bright quiet man” whom the book describes as “smiling like a great friendly fish” — albeit one with muttonchops — and blowing a tuba while parading around a backyard barbecue.

Joe Houston Kesey was born on June 18, 1938, in La Junta, Colo. According to family lore, he was nicknamed Chuck when Ken, who was about two and a half years older, exclaimed that his brother “sure is chuckleheaded.”

Their father, Fred Kesey, Wolfe wrote in “The Electric Kool-Aid Acid Test,” was one of the “Protestant entrepreneurs who looked to the West Coast as a land of business opportunity.” Moving west with the family, he managed a prosperous cooperative creamery in Eugene. Their mother, Geneva (Smith) Kesey, maintained the home.

Fred Kesey started his sons “shooting and fishing and swimming as early as they could in any way manage it,” Wolfe wrote, “also boxing, running, wrestling, plunging down the rapids.”

In 1960, Chuck Kesey graduated with a degree in dairy science from Oregon State University. That same year, he married Susan Jameson, who had a secretarial degree and a talent for numbers.

For $150 a month, they leased the defunct Springfield Creamery plant in the Willamette Valley of Oregon and began bottling milk in glass jugs under the Springfield name. Between books, Ken Kesey worked briefly at the creamery.

In 1972, when the business was struggling, Chuck Kesey enlisted the Grateful Dead, his brother’s friends, to stage a benefit concert. The event, in Veneta, Ore., drew about 20,000 people and made enough money to restore the creamery to solvency. (A documentary film about the show, “Sunshine Daydream,” was screened periodically for years and officially released in 2013.)

As the company tells it, Ms. Van Brasch Hamren had long been making yogurt at home before becoming the bookkeeper at Springfield Creamery. She shared her technique with Mr. Kesey, who thought of adding a bacterial ingredient that he remembered from his college dairy science studies. At a moment when Americans were awakening to the benefits of natural foods, probiotic yogurt was a hit.

Because Ms. Van Brasch Hamren answered the phone, “Springfield Creamery, this is Nancy,” customers began asking simply for “Nancy’s Yogurt.” It became the company’s signature brand.

“They were in on the bottom floor of the natural food movement,” Sheryl Kesey Thompson said in an interview. “We kids would always beg for sugared cereal and Pop-Tarts, but our parents would never buy them.”

Mr. Kesey’s wife died in August. In addition to his daughter, he is survived by his son, Kit Kesey, five grandchildren and five great-grandchildren.

Nancy’s Probiotic Foods is still family-owned, run by Mr. Kesey’s children and several of his grandchildren. While the company admits to having been slow to catch on to the Greek yogurt boom around 2010 — only launching its own variety a couple of years later — it has expanded at a reasonable pace, valuing its independence more than accelerated growth. The world, meanwhile, has caught up to its innovations.

“Fifteen years ago, we didn’t put the word ‘probiotic’ on the Nancy’s label because nobody knew what the word was,” Ms. Thompson told Oregon Business magazine in 2017. “Now it’s our core message — our products help regain digestive health.”