The Children's Hospital Medical Center in Boston-a teaching associate of the Harvard Medical School-fired 18 striking security guards yesterday.

"The security officers who walked out demonstrated, by leaving their posts and later by attempting to halt delivery of medical supplies, food and other materials, that they were putting their own self-interest ahead of the interests of the children here," hospital spokesman William J. Brennan said last night.

Twenty-two of the 28 security guards employed at the hospital had walked off from their jobs October 16 and had maintained an obstructive picket line at the medical center. They were demanding that the hospital allow them to unionize and that they be permitted to carry guns.

"The men have had a full week to reconsider their action-an action which was illegal in the first place," Brennan added. The hospital has been contending that the policemen were violating a state law which prohibits nonmusical hospital employees from striking.

A spokesmen for the striking policemen said that the picket lines would continue indefinitely. He disagreed with Brennan's claim that essential medical supplies were being blocked, saying that the pickets were only obstructing delivery of non-essential items such asstationery and food for the staff cafeteria.

Brennan said that the hospital administration expected no retaliatory action from other staff members. "The few workers I talked to personally felt we had made the correct decision," he said.

Both sides in the dispute have filed unfair labor practices complaints with the Massachusetts State Labor Relations Commission. A hearing will be held Oct. 29.