LOS ANGELES (AP) _ A ''fourth and final suspect'' was arrested today in the killing of two college students, according to police, who said two suspects were on probation from earlier crimes when the couple was abducted and shot.

Donald Roy Bennett, 21, was arrested at his South Central Los Angeles home at 12:05 a.m. and booked for investigation of murder in the killings, said police spokesman Sergio Diaz.

Diaz said detectives described Bennett as the last suspect in the killings of UCLA student Michelle Anne Boyd, 18, and California State University- Northridge student Brian Edward Harris, 20, who were abducted near the UCLA campus in Harris' car on Sept. 30.

Their bodies were found Sunday in a field near the San Diego Freeway.

Three other men booked for investigation of murder, Stanley B. Davis, 23, Deandre Brown, 21, and Damon Redmond, 19, are being held without bail.

All four men were identified as gang members.

Police said Davis, Brown and a third, unidentified individual were involved in an incident similar to the Harris-Boyd case.

In May 1984, UCLA student David Kingsmill, then 21, was leaving his car in the Westwood area when three men abducted him at gunpoint. They took him to a field, robbed him, stole his car and left him there, unharmed.

Later that month, sheriff's deputies arrested Davis and Brown after they were stopped in Kingsmill's car, city attorney's spokesman Ted Goldstein said

Because Kingsmill could not identify either man as his abductor, Davis, who was driving the car, was prosecuted for misdemeanor counts of joy-riding and receiving stolen property, Goldstein said. He was convicted and sentenced to 60 days in jail and placed on 11/2 years' probation.

Brown pleaded guilty to a reduced charge of auto theft and received an unspecified probation term, police said.

''If they had really gone to jail for a year or two, then these kids might still be alive,'' said police Cmdr. Larry Binkley said.

Police believe the new case occurred because the alleged gang members' truck broke down and they wanted a vehicle.

''They were going to commit a robbery that night and use that car,'' Binkley said.

Police believe the couple was killed with an Uzi automatic or semi- automatic rifle so they could not identify their abductors.

Harris' Honda was found doused with gasoline and burning behind a Firestone-area auto parts store.

Redmond's fingerprint was found on the side-view mirror of the Honda, Binkley said.