A fire swept through a three-story house in Old Town Alexandria early yesterday, seriously injuring four family members and a passer-by who tried to rescue them.

Firefighters pulled two young girls from the second floor of the house at 613 S. Alfred St. and one, 18-month-old Jennifer Mathers, was listed in critical condition last night at Children's Hospital in Washington. Her 3-year-old sister Melinda was in serious condition.

Their parents, Stephen and Nancy Mathers, and a man who tried to save them also suffered serious injuries, the fire department said.

The blaze was reported at midnight, 15 minutes after a cigarette apparently ignited a first-floor couch. Stephen Mathers, 36, who was listed in serious condition last night at Alexandria Hospital, was burned as he tried to put out the fire.

His wife Nancy, 29, jumped from the house to try to get help for the children. She suffered smoke inhalation and severe spinal injuries and was listed in serious condition at Alexandria Hospital.

Joseph Kenny, 21, of Fredericksburg, who was walking past the house and saw the flames, jumped through a window to help the victims, fire department officials said. He was in serious condition at Alexandria Hospital with first- and second-degree burns.

Investigators said Stephen Mathers tried to douse the flames with tap water after he heard a fire alarm sound. He tried to push the couch through the front door but it became jammed, preventing firefighters from entering.

" By the time we got there the smoke was everywhere," said Deputy Fire Marshal Stephen Flynn.

Neither child was breathing when firefighters arrived, Flynn said.

Fire department officials put the damage to the house at $90,000.