One by one, dozens of young men anxiously paced outside Circuit Court Room Three at the courthouse in Upper Marlboro yesterday, amid the clatter of nurses setting up blood testing equipment and the piercing sounds of babies crying in toy-littered marble hallways.

As their names were called, the men were led before a judge to enter a plea, and then whisked downstairs to have a needle jabbed in their arms.

This is paternity court, Prince George's County style.

Every Friday, scores of mothers tote babies into the courthouse, where four courtrooms, two judges and a court master are devoted to deciding paternity. Up to 120 blood tests are given on the spot by attendants from Johns Hopkins Hospital under contract to the court.

This conveyor-belt judicial procedure, begun in January, has reduced to hours paternity disputes that previously lingered up to four years. Now only difficulties in arranging visitation, child support or other payments lengthen the process beyond 10 or 15 days.

Much of the credit for the stepped up schedule is because of the decision by Judge David Ross to allow the blood testing to be done in the courthouse. The county has a contract with the Maryland Rh Typing Lab of Baltimore, an affiliate of Hopkins, to administer the three Human Leukocyte Antigen (HLA) blood tests necessary per case. Previously, taking mother, child and the accused father to a hospital for blood testing meant months of delay.

Matching blood types among mother, child and father is generally considered the supreme test of responsibility in paternity cases. HLA tests lead to identification 99 times out of 100, according to blood typing labs. As of July 1, it is accepted for trial evidence in Maryland.

HLA blood tests achieve such accuracy by identifying genetic markers called antigens in an individual's white blood cells, according to Dr. Lamya Alarif of the Washington Hospital Center. She said these antigens are more precise than fingerprints. A child receives exactly half of his or her gene structure and antigen level from the mother and half from the father.

The Prince George's approach is so new that court officials throughout the Washington area and across the country have called to learn how the system works, Ross said. He said at first most express disbelief that such a specialized process can work and work quickly, but nearly all of the callers want to know how they can use it.

The system has won praise from legal interests as diverse as the Maryland Bar Association and the American Civil Liberties Union.

"It simplifies everything and solves problems for everybody involved in a paternity action," said Richard P. Arnold, who works in the state attorney general's child support collection office in Prince George's County. "Our caseload has nearly doubled because of it."

In most civil courts, paternity cases are wedged in judges' caseloads whenever possible. Generally less than a full day per month is given to paternity cases and large backlogs are common.

"They were definitely a low priority because if we had to choose between tying up a court for a murder one trial or a paternity suit, we would always pick the murder," said Ross, one of two judges assigned to hear paternity cases. "It used to be that we'd only touch them after everything else fell out. But then the number of paternity cases skyrocketed. We had to come up with a new approach."

The new system has significantly eased the backlog, Ross said, but "we still have so many paternity cases that we're unable to count them . . . . So far we have gone from about three years of backlogged cases to about eight months. By the first of the year we should be close to catching up and we should be able to get everything done on a case in 10 to 15 days."

Judge Ross says the arrangement is a good deal for all concerned. "The hospital which charges $450 for the set of three tests stands to make a bundle off it, and the county doesn't have to foot the bill. Whoever loses pays. Occasionally the state will pay for the child's test if the mother is on welfare."

The blood typing contract for the court has become so lucrative that competing firms are jockeying to provide cheaper tests. Already, so many blood tests are needed that the attendants frequently run out of needles.

It's not always a matter of proving who is the father. The blood tests also have uncovered instances where people fraudulently claimed children in the hope of collecting extra welfare benefits. In one case, Ross recalled, the "father" was not the father, the "mother" was not the mother and the baby was not their baby. The man and woman were living together and borrowed a baby from neighbors in an attempt to have the child legally proved their own. The blood test showed there was no relation between any of them.

Once, the judge said, a fight developed in a courtroom between two or three men who all claimed to be the father of a baby girl. All had been involved with the mother and all thought they would soon be moving in with her. But once the blood test results came back, the father was named and the other two were charged with welfare fraud.

But most of the time, it is one man trying to avoid the costs of fatherhood. A 25-year-old construction worker named Anthony, who a week ago came to the courthouse denying he was the father of a 3-year-old boy, said "when I first heard she was pregnant, some of my friends told me 'hey, don't worry about it, paternity is a breeze to avoid.' " But before the end of the day, he said, "man this sure hasn't been a breeze . . . no sir, I've been caught, there's no avoiding it now."

The word of the effectiveness of the test has spread, so that in three or four of every 10 cases, the accused men avoid the cost of the blood tests and plead guilty right away. One of those who chose that route yesterday was Tommy, 23, and unemployed.

Twenty-year-old Lisa, an old girl friend of Tommy's, sat staring ahead, not looking at him when he entered the courtroom. He didn't look at her either. But Tommy did look at the screaming 2-year-old boy sitting next to her, and when the judge asked him how he would plead, Tommy said guilty.

The boy stopped screaming, got away from Lisa and scampered to the witness stand and climbed on it. The judge pointed to the boy, and asked Tommy if he recognized the child. Tommy mumbled "yeah" and then blurted "He's a good little boy . . . he's my little boy . . . I'll take him." Now he was a father.