WASHINGTON (AP) _ James D. Briley, one of three brothers convicted of murder after a 1979 crime spree left 11 people dead, today lost a Supreme Court appeal aimed at sparing him from Virginia's electric chair.

The nation's highest court let stand Briley's death sentence for the Oct. 19, 1979 murders of a Richmond woman and her five-year-old son.

Briley is scheduled to be executed April 18.

His brother, Linwood, was executed last year. Anthony Briley, who was 16 when arrested in 1979, is serving a 109-year prison sentence.

Linwood and James Briley participated in the largest death row escape in U.S. history last year, in which six inmates broke out of the Mecklenburg Correctional Center near Boydton, Va.

The Brileys were the last to be recaptured. They were arrested in Philadelphia last June 19.

James Briley was sentenced to death for murdering Judy Barton and her young son, Harvey, at their home in Richmond. Mrs. Barton's husband, Harvey Wilkerson, also was killed.

The appeal turned away today did not challenge Briley's convictions, but contended that he was improperly sentenced to death.

The appeal argued that jurors were not given a ''clear explanation'' of how they should consider mitigating circumstances - those factors suggesting that life in prison, and not death, would be the appropriate punishment.