An FBI fiber expert testified today that fibers from the bodies of 11 slain black youths matched those taken from the home and car of Wayne B. Williams. Included, he said, were two human hairs found inside the shirt of one victim matching those of Williams.

The human hairs, taken from the shirt of Patrick Baltazar, 11, exhibited the "same microscopic characteristics" as hair samples from Williams' head, said FBI agent Harold Deadman, a chemist who has spent six months testing the fibers.

While hair comparisons cannot prove positive contact between two individuals, Deadman said he had "rarely seen instances where hairs from two different individuals exhibit the same characteristics."

Williams is not accused of killing Baltazar. He is standing trial only for the murders of Nathaniel Cater, 27, and Jimmy Ray Payne, 21, two of 28 young blacks found slain during a 22-month period that began in July, 1979.

Deadman's testimony today was the first mention that human hair--possibly that of Williams--was found on one of the victims, as prosecutors used that and substantial new fiber evidence to broaden their case against the 23-year-old free-lance television cameraman and self-styled talent scout.

Last week, Judge Clarence Cooper allowed prosecutors to introduce evidence from 10 of the other killings in an attempt to show that they fit a pattern similar to that in the murders of Cater and Payne.

Deadman's fiber and hair testimony is among evidence linking Williams to nine of the slayings and that of a 10th victim never added to the investigative task force's list.

Ten witnesses have placed Williams with six of the victims, although he had denied at a news conference last June that he knew any of them. The jury has heard a tape of that conference.

Under withering cross-examination by defense attorney Alvin Binder, Deadman conceded that two Caucasian hairs were found on one victim, but he called them irrelevant. What led him to believe the victims had come into contact with Williams, Deadman said, was the sheer amount of fiber and hair matchups.

"Each additional association adds to the significance tying 11 victims to Wayne Williams' home or objects in his home or car," Deadman said.

In addition to the human hairs, Baltazar's body yielded the largest amount of fiber evidence. Eleven types of fibers and dog hairs matched material from Williams' home and car, the lining of a gray glove and leather jacket, a bedspread, a yellow blanket and the carpet of his 1970 station wagon. Three fibers from vacuum sweepings of Williams' car also matched fibers found on Baltazar's clothes and body, Deadman said.

Deadman said fibers from the victims were "consistent with" other items belonging to Williams, including his bedroom carpet, two bedspreads, a blanket, a bathmat, a kitchen carpet, carpeting from a workroom in the house and the felt trunk lining of a 1979 car once owned by the Williams family.

Deadman has conceded that fiber matches are circumstantial and cannot be used alone for conviction. But he said the presence of fibers on 11 of 12 victims resulted through contact either with items in the Williams home or in one of three cars he drove.

"I consider the 11 victims to be related," he said.