A Prince George's County Circuit Court jury early yesterday convicted a Northwest Washington man of conspiracy to commit murder in the 1979 contract slaying of a young Suitland woman, but acquitted him on a charge of first-degree murder in the case.

Serrano Coward Jr., 23, could receive a life sentence for conspiracy to commit murder, the same sentence that is often given for first-degree murder. He will be sentenced next month.

Coward is the third man to be charged in connection with the slaying of 21-year-old Patty Poellot. Poellot's husband Ray, 23, and another man, James R. Hamilton, 22, of Southeast Washington pleaded guilty to the murder in 1979 and are both now serving life sentences. All three men previously worked as security guards for the Wackenhut Co.

Both Ray Poellot and Hamilton were brought to court in Prince George's County last week to testify in Coward's trial. Poellot told the jury at his trial that he asked Hamilton to kill Patti because he believed she was seeing other men. Poellot testified that he promised to give Hamilton $5,000 from Patti's insurance policy.

Hamilton testified that he recruited Coward to the scheme by promising him $2,500. He told the jury that he held Patti on her bed while Coward stabbed her.

Coward took the stand in his defense on the fourth day of the week-long trial, and denied having anything to do with the killing.

He testified that he accompanied Hamilton to a bar near Poellot's apartment, the Lamp Post Inn, on the night of the murder and that Hamilton left him alone in the bar for 45 minutes. Then Hamilton picked him up from the bar and drove him home, he told the jury.

The jury deliberated more than 11 hours before reaching its verdict. Members of the jury refused to comment afterward on why they chose to convict Coward on the conspiracy charge rather than the first-degree murder charge.

Coward was arrested by FBI agents in February. County police had been searching for him since the slaying occurred in April 1979.