A 27-year-old Southeast Washington man will go on trial today in Prince George's Circuit Court in the fatal shooting last February of Antonio M. Kelsey, an off-duty county police officer.

Defendent Steven Baines is accused of first-degree murder for allegedly shooting Kelsey, 22 with the officer's own gun. If convicted, Baines could face the death penalty.

On Feb. 2, according to police accounts, Kelsey was working in uniform as a security guard at Cox's liquors in Landover, when a man walked in with a woman, displayed what appeared to be a bag of marijuana, and taunted the officer, saying, "What are you going to do about it?"

Kelsey chased the man up the street, and radioed Seat Pleasant police headquarters that he was pursuing a man on foot. Policemen who rushed to Kelsey's aid found him sprawled on a patch grass near the store, shot in the head.

Kelsey, the first black county officer killed in the line of duty, was permitted to work as a guard off duty under police regulations.

The trial, expected to last at least four days before Circuit Judge Vincent J. Femia, will pit county State's Attorney Arthur A. Marshall against noted defense counsel R. Kenneth Mundy, who will head a team of lawyers.

Marshall, who vowed years ago to personally prosecute every murder case involving a police officer, faced Mundy in the controversial 1979 trial of Terrence G. Johnson. Johnson, a black teenager who shot two white Prince George's police officers in 1978, was convicted of manslaughter in one slaying and found innocent by reason of insanity in the second. The case greatly increased racial tensions in the county.