The end of an era came early yesterday as milkman James Johnson pulled out of Rockville about 5:30 a.m. in a shiny white truck loaded with Sealtest dairy products.

For seven years Johnson had cruised Washington's narrow side streets and Montgomery County's commuter-clogged roads, delivering half-gallons of fresh milk, quarts of orange juice and bacon and butter to dozens of residents including a D.C. Superior Court judge in Northeast and 60 senior citizens in Wheaton.

But such doorstop deliveries ended when Damon Corp. of Rockville--the only distributor still offering area-wide home delivery of milk and other dairy products--decided to stop such service yesterday.

"It's a damn shame, pure and simple," Johnson, 45, said yesterday morning as he hefted wire milk crates in the refrigerated compartment of his panel truck. "I really don't want to tell people that this is the end. Some of them are angry."

Sylvia Byrne was one of those who was upset. "It's the end of an era of nice service," said Byrne, an Arlington resident whose milk had been delivered for the last 21 years.

She learned about the home-delivery cutoff yesterday morning when she telephoned the Damon Corp. with her orders for the coming week, she said.

"It was more than a lovely convenience," continued Byrne, 45, a federal worker who owns no car. "It was sort of like the way things used to be in the old days."

The end of the Sealtest deliveries also marks the demise of a once-flourishing corps of milkmen, neighborhood regulars nearly as popular as their wares. A one-truck firm in Laurel still supplies some Maryland homes with milk, but has never compared with the fleet of Washington milkmen who used to serve more than 125,000 customers in their 13-hour days.

Twenty-five years ago there were 10 dairies in the Washington area. In those days, milk processed by the likes of Thompson Honor Dairy, Shenandoah's Pride and Sealtest was bottled, usually topped with sweet cream. "The quality was superb, not like this inferior stuff," said Johnson, nodding toward neat racks stocked with paper cartons. "You gotta wonder what they put in it these days."

With the spread of supermarkets and convenience stores and sharp rises in fuel and distribution costs, the home deliveries that once accounted for over half of total milk sales soon dried up, dropping to 21 percent by 1968. Thompson Honor, the area's oldest and largest home-delivery dairy, closed in 1971 after 90 years in operation; Shenandoah's Pride, which served thousands of Northern Virginians for nearly 50 years, stopped home deliveries in 1979.

According to the Milk Industry Foundation, which represents most of the nation's milk processors, home deliveries now occupy less than 3 percent of the market.

No longer will Johnson brave impassable roads, as he did during the January blizzard to get milk to his 120 daily customers. Nor will he chat any more with the elderly diabetic woman in Maryland who depended on him for orange juice and occasional gossip.

Earlier this week Lawrence R. Paul, Damon's general manager, told 20 deliverymen, including Johnson, and one office worker that they would be laid off when the 5,000 weekly home-deliveries ended yesterday. Paul, who spent Friday negotiating with the workers' union representatives, said Damon's residential customers had been notified by mail that the deliveries would be eliminated. (The firm's five remaining drivers are expected to stay to serve Damon's wholesale customers, according to employes.)

Lillian LaRocque of Northwest, one of James Johnson's customers, said she was "utterly amazed and shocked that the company gave no notice that they were cutting off service." LaRocque and her husband, retired Navy Adm. Gene R. LaRocque, had been Thompson customers before switching to Sealtest.

So had Superior Court Judge Luke Moore and his wife, Dorsey, who ordered their milk for 33 years. "I realize that things change, but I'm flabbergasted by this," said Dorsey Moore, who lives in Northeast.

"The deliveries were something I took for granted as an adult," she said. "They were always there."

Richard Kearns, a 55-year-old Damon driver who got his first milk route at age 11, said the 12- and 14-hour hour days he and other milkmen regularly worked "were hard, but worth it."

"There's no other job like it," said Kearns, who worked for Sealtest since 1949 and whose father was a driver for Thompson Honor Dairy for 24 years. "But the good ol' days when people wanted the milkman are gone."

Because of disagreements with Damon Corp.'s managers, Johnson said he will probably leave the dairy products business.

"Two years ago we were prosperous, and now look at us," said Johnson, who worked at Thompson Honor Dairy for eight years. "This amounts to one big inconvenience for a lot of people." Johnson said he may return to school to study refrigeration.

Meanwhile, Ray W. Pettit, the owner of The Milkman, a small milk delivery firm in Laurel, said he will pick up as many of the Damon Corp. customers as he can. Pettit's firm, a one-truck operation founded in 1973, now serves 400 households in the Bethesda-Chevy Chase area and 15 homes in the District.