An explosion yesterday in a laboratory at Georgetown University injured four people, three of them firefighters, officials said.

The explosion occurred just before 8 a.m. on the top floor of the Reiss Building, a six-story science facility in the 3800 block of Reservoir Road NW.

A security guard and three firefighters were trying to extinguish a smoky fire when there was a "small explosion," said the center's spokeswoman, Kate Egan. District fire officials said the blast apparently occurred when water being used to put out the fire came in contact with sodium, a combination that can be explosive.

The fire department was alerted at 7:42 a.m. when an automatic alarm in the building went off. The response eventually drew more than 75 firefighters, a hazardous materials unit and the police department's bomb squad.

The three firefighters are John T. Zabasky, 50, Archie Jefferson, 20, and Darnell E. Irving, 35. Jefferson suffered damage to one of his eyes and Zabasky was treated for smoke inhalation, Egan said. Details on Irving's injuries were not immediately available.

The three and the security guard, Zachery Gaston, 30, were in stable condition yesterday at Georgetown University Medical Center.