Derrik O. Holmes, a former contender for the world bantamweight boxing title, was sentenced yesterday by a Prince George's County Circuit Court judge to life in prison, with all but 35 years suspended, for robbing and attempting to murder a Christmas tree salesman in a Clinton lot in December 1983.

Holmes, 29, of Adelphi, was also sentenced to 20 years in prison for robbery with a deadly weapon and to 15 years for use of a handgun in commission of a violent crime, both to be served concurrently with the life sentence. He will have to serve at least 11 1/2 years before becoming eligible for parole.

Holmes was convicted by a jury last November after an emotional three-day trial. During the trial, Roswell C. Howard, 40, of Owensboro, Ky., testified that Holmes came to the van on the Christmas tree lot where Howard slept, robbed him and shot him four times.

Holmes said throughout the trial that he was innocent, testifying that he was with his girlfriend the night of the incident. Holmes also told Circuit Court Judge Arthur M. Ahalt he was innocent before receiving his sentence.

Defense attorney Hal Sullivan said yesterday that Holmes plans to appeal the case. "It was a tragic incident for the victim and now it is for Derrik as well," Sullivan said after the sentencing.

Prosecutor William Shockley said he was satisfied with the sentence.

"I thought the judge was fair," Shockley said. "It's difficult for the court to impose such a sentence on someone, like Holmes, who doesn't have a prior violent crime record."

During the trial, Howard testified that Holmes worked on the Christmas tree lot the weekend before the shooting and that he and Holmes became "good friends." Howard said Holmes came to the lot about 10:45 p.m. Dec. 20 with another still-unidentified man, asked for the money Howard had made selling trees and then shot him with the gun Howard kept in the van for security.

Holmes testified that he and his then-girlfriend Nancy Lee met the night of the shooting and were together until the next morning. Nancy Lee corroborated Holmes' testimony. Lee's sister-in-law also testified that she heard him in the house that night about the time of the shooting.

The jury deliberated two days, with evening recesses, before reaching its verdict.

In August 1980, Holmes challenged 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.