A mother rushed into her blazing home in Rockville last night and rescued her 14-year-old son after a 20-pound propane gas bottle stored inside exploded.

The mother, Lillian Jancs, and her son, who had been set afire in the explosion, escaped by plunging through a living room window onto the lawn as flames spread quickly and destroyed their two-story, colonial-style home.

Two other Jancs children had gone outside along with their mother a few moments earlier after the gas bottle -- being kept beneath a stairway -- began leaking about 7 p.m. They suffered minor injuries from the exploding gas and flames.

Moments before the explosion, Mrs. Jancs had been in the front yard of her home at 3502 Sloan St., calling her son to come out, a neighbor said.

"She said, 'Jimmy come outside. Jimmy come out.' Then the whole thing just blew," said Elaine Graydon, whose house is next door.

"We both pulled (the children) away and ducked down. I felt the glass go shooting past me. I thought Lillian and Jimmy were both dead. She had gone back into the house. The livingroom windowpane fell out and Jimmy and Lillian came leaping out the window. He was on fire."

"Within 30 seconds the entire house was in flames," said another neighbor, John A. Conway.

Another neighbor, Mark Tarling, 16, said he dashed outside after the explosion.

"There was shouting and screaming. The children were on the lawn. I just got (Jimmy) on the ground and put the flames out with my hands," Tarling said.

Mrs. Graydon said she had gone to the Jancs' home after Jimmy had come to her house to ask for her husband, who was not at home.

"He said the gas fell over and he wanted to see if he (her husband) could help. I grabbed my coat. Lillian and the children were outside. I started talking to her, and Jimmy went inside. I could hear this hissing sound all the way out into the street," she said.

While fire officials would only give sketchy details of the incidents neighbors said Mrs. Jancs was home with three of her four children while her husband, F. William Jancs, and another son, Tommy, 13, were attending a meeting at St. Jude's Catholic Church.

Neighbors quoted the father, a Food and Drug Administration investigator, as saying the gas bottle recently purchased for a family camper that also was destroyed in the fire, accidentally tipped over when one of the children went to the storage closet.

There was no immediate explanation why the bottle, which has a safety valve, exploded.

Mrs. Jancs was admitted to Holy Cross Hospital, suffering from a broken and burned left arm. Her son, Jimmy, was in serious condition at Children's Hospital with second- and third-degree burns over 50 percent of his body. The other two children, Jeanne, 8, and Kenny, 10, were treated at Holy Cross and released to neighbors who were keeping them for the night.