A 21-year-old black man who became the 26th victim in Atlanta's string of slayings probably died of asphyxiation, as 14 others did, a medical examiner said yesterday. Authorities worried that his death reflects a shift to older victims.

The body of 21-year-old Jimmy Ray Payne, last seen April 22, was found Monday floating in the Chattahoochee River. He was the fourth adult victim of slight build in the series of murders.

Assistant Fulton County Medical Examiner Saleh Zaki said he had made a "working diagnosis" of death by ashphyxiation in Payne's case by excluding other causes. To determine a definite cause, he said officials would need lab data that would not be available for several days.

The case is "very similar to some of the other cases," 14 of which were ruled asphyxial deaths, Zaki said.

He said there was no evidence of external injuries, sexual molestation or a struggle.

Payne was about 5 feet 7 and 130 pounds. The last three young adults found dead in the Atlanta area also were in their early 20s and of slight build.

Public Safety Commissioner Lee Brown said Monday night after Payne's body was found that policer were worried about the change in pattern to adults because "we now have four victims of homicides. Previous to that, all the victims were under 16, the youngest being 7."

All the adult cases have similarities, including their stature and the fact that the bodies were found in rivers, he said.

Meanwhile, police found two missing young blacks who fit the profile of the latest victims. The cases were handled by the missing-persons unit rather than the special task force in charge of the 26 murders and one disappearance of black youths since July 1979.

Last night, police found Richard Character, a 33-year-old black man reported missing Monday by his mother, who said he hadn't seen him since April 14. Police found Character in a downtown park, where he said he had been staying for the past few days.

For a few hours, police also were looking for 21-year-old Herman Pittman, whose mother said he had failed to return home Monday after going out for a beer. But Pittman, who stand 5 feet 6 and weighs 140 pounds, called home from work yesterday after hearing that he was thought to be missing, police said.