Derrick O. Holmes, a former contender for the world bantamweight boxing title, was found guilty yesterday in Prince George's County Circuit Court of robbing and trying to murder a Christmas tree salesman on a Clinton lot last December.

The jury began deliberating Wednesday afternoon and reached its verdict about 1:45 p.m. yesterday. It had recessed Wednesday and Thursday evenings.

Holmes, 29, was convicted of first degree attempted murder, robbery with a deadly weapon and use of a handgun in commission of a violent crime. He could receive a maximum penalty of life in prison.

Holmes, of 7900 Kreeger Rd. in Adelphi, appeared calm while the verdict was read. His mother began to cry after Holmes was escorted out of the courtroom in hancuffs.

"I know my son is innocent," she said.

Holmes denied the charges during the emotional three-day trial. Crying as he testified, he said that he was with his girlfriend the night of the shooting. The former girlfriend confirmed his testimony in court, and three other witnesses testified they had seen him that night.

The salesman who was shot, Roswell C. Howard, 40, of Owensboro, Ky., told the jury Holmes worked at the Christmas tree lot the weekend before the shooting and they "got along real well."

Howard testified that on Dec. 20 about 10:50 p.m. Holmes and another still-unidentified man came to the van on the lot and talked Howard into showing them a gun he kept in the van for security. Howard said Holmes took the gun and demanded the money Howard made from selling trees that night. Howard said Holmes shot him four times.

He said that after the two men fled he hid in the Christmas trees fearing they would return, and then ran to a nearby Hardee's restaurant where employes called police.

Holmes testified that he and his then-girlfriend Nancy Lee met about 9:45 p.m. at a New Carrollton restaurant and went to two convenience stores and to pick up a bag of marijuana. Holmes said they arrived at the Hyattsville home of Lee's sister-in-law about 10:45 p.m., where he slept until 7:30 the next morning.

Although a warrant was issued for his arrest the day after the incident, Holmes did not surrender to police until Jan. 9.

In August 1980, Holmes fought super bantamweight champion Wilfredo Gomez for the world title, but was knocked out in the fifth round. During the trial Holmes said he fought his last of 22 professional fights in 1982 and had a career record of 139 victories, 10 losses and 75 knockouts.

Holmes will be sentenced Nov. 21.