Bloodstains taken from a white 1970 Chevrolet station wagon Wayne B. Williams drove about the city match the blood types of two young murdered blacks, a state crime lab expert testified today.

Linda D. Tillman, a forensic serologist who swabbed down the black vinyl back seat after it tested positive for human blood, said she detected two stains that matched the blood type of William Barrett and another spot that matched that of John Harold Porter.

Barrett, 17, was found asphyxiated on May 12, 1981, with post-mortem stab wounds, according to a medical examiner. Porter, 28, stabbed to death and found April 12, 1981, was never added to the official task force list of 28 murder victims.

Both are among the 10 young black victims prosecutors have been allowed to introduce in an attempt to demonstrate a "pattern, scheme or bent of mind" on the part of Williams, who has been charged with the deaths of Nathaniel Cater, 27, and Jimmy Ray Payne, 21.

"Would it be possible for that blood to belong to Wayne Williams?" asked Assistant Fulton County District Attorney Gordon Miller.

"No, it would not," said Tillman. Barrett belonged to the group A blood type and Porter to type B, while Williams' blood is type O, she said.

Tillman, who knelt over the tattered car seat to show rapt jurors where she had taken the blood samples, said the types of another stain on the seat and another on the trunk floor were impossible to determine.

The detection of blood enzymes in the two different samples indicated the stains were less than eight weeks old when taken from Williams' car on June 4, 1981, testified John G. Wagel Jr., another crime lab serologist. Both victims were found within eight weeks of their deaths.

He put the odds of someone having the same blood group and enzyme as Porter at 7 out of 100, while Barrett's was less rare, 1 in 4.

"We object to this Hollywood show!" barked defense attorney Alvin Binder. On cross-examination, Tillman said it was "impossible" to identify blood as coming from a victim, even if blood groups matched.

The bloodstains appeared to be part of an overall prosecution strategy to use every bit of evidence in its cupboard, no matter how circumstantial. Witnesses have placed Williams with six victims, and an FBI fiber expert testified Monday that hundreds of fibers and hairs found on the bodies of 11 victims link them to Williams because they match materials from his home, car and dog.

A slapjack, found in Williams' ceiling, was also introduced as a possible murder weapon Monday. Eric Middlebrooks, 14, died from a blow to the head.

Even the charred remains of photographs found in Williams' barbecue pit were trotted out just before court recessed today, hinting at possible destruction of evidence linking Williams to his alleged victims.

"You don't normally burn photographs in your barbecue pit, do you?" said a prosecutor. "It makes the hot dogs taste funny."

At one point in the trial, in trying to get his client's attention during testimony on hair evidence, Binder startled the largely black jury, as well as the black judge, Clarence Cooper, by addressing Williams as "Wayne, boy."