Five guards at Lorton Reformatory were slightly injured last night during a fight with five maximum-security prisoners in a lavatory of the prison adminstration building, D.C. corrections officials reported.

One inmate also was hurt in the 7:30 p.m. incident and was taken to D.C. General Hospital "as a precaution," but it was believed that his injuries also were not serious.

A prison spokesman said the fight broke out after a prison lieutenant, who had been searching for an inmate reported missing from his cell, was jumped by the prisoners in the lavatory. A prison captain came to his aid, then about a dozen more guards joined in as the melee spilled into an adjacent hallway, the spokesman said.

The prisoners, some of whom were armed with homemade knives, were subdued after a few minutes, according to the spokesman. The four uninjured inmates have been placed in isolation cells pending an investigation.

It was unclear last night why the prisoners--all serving life sentences--were absent from their cells or how they entered the building, which is a short distance from confinement sections of the prison and had been closed for the evening.

The spokesman said each of the inmates might have had prior authorization to be elsewhere in the prison complex at the time of the incident, but none should have been in the administration building.