Invoking the Fifth Amendment's guarantee against double jeopardy, a Fairfax County General District Court judge dismissed yesterday trespassing charges against four antiabortion demonstrators.

The four, who were arrested for trespassing at the Northern Virginia Women's Medical Center on July 20, had on July 27 been fined $500 each and given 60-day suspended jail sentences for contempt of court by a federal judge in connection with the same incident.

Judge John A. Rothrock Jr. ruled yesterday that trying the protesters for violating the county's trespassing ordinance would, in light of their plight in federal court, violate the U.S. Constitution by subjecting them to two trials for the same offense.

The protesters were found guilty of contempt in July by U.S. District Court Judge J. Calvitt Clarke Jr. for violating a court order restricting protests at the clinic. Earlier this summer, Clarke issued the injunction said to be the first of its kind in the nation, forbidding the demonstrators to enter the clinic.

Deputy Commonwealth's Attorney Steven A. Merril, who argued that prosecution of the four protesters on trespass charges did not constitute double jeopardy, said after the dismissal that the judge's decision "does have some legal merit."

Last spring, two Fairfax General District Court judges dismissed charges against other antiabortion protesters who trespassed at the clinic, ruling that the protesters had a right to trespass because they were attempting to save lives of unborn children.

After the tiral, David Gaetano, 27, one of the protesters, said he would "rather have been acquitted on the substance of our protest - that we were in there to protect human lives."

Commonwealth's Attorney Robert F. Horan Jr. said yesterday that any future trespassing incidents at the clinic will probably be dealt with first by federal courts because Clarke's injunction is still in effect.

In addition to Goetano, those facing charges yesterday were Diane Bodner, 19, a University of Maryland student; Jeanne Marie Miller, 18, a Yale University student from Bowie; and Mary Beth McKernan, 18, a Bishop Denis J. O'Connell High School graduate from Arlington.