Prince George's County health officials closed an Oxon Hill restaurant last night after 31 people who had dined there came down with an illness that was diagnosed as salmonella poisoning, a Prince George's County health officer said.

Eleven of the 31 were hospitalized, although Dr. Helen B. McAllister of the county health department said none of the cases was thought to be serious.

McAllister said the Ranch House Restaurant, at 6355 Oxon Hill Rd., which specializes in steak dinners, was closed indefinitely about 8 p.m.

She said a large number of salmonella cases turned up yesterday, and most of those diagnosed as having the disease were experiencing diarrhea and vomiting.

In each case, the person had eaten at the Ranch House May 30 or May 31, McAllister said.

At this point, health officials have not been able to determine the exact source of contamination, McAllister said. A team of inspectors went through the restaurant last night and was expected to return today. McAllister said it is possible that the food was infected before it reached the restaurant.

Only four of the food poisoning cases had been reported before yesterday, McAllister said, and the sudden appearance of a number of cases is characteristic of the disease. "The onset is usually sudden and includes headaches, cramps, diarrhea, sometimes vomiting and fever."

Salmonella is a bacterium that infects the gastrointestinal tract and can be acquired from food, contact with infected people or ingestion of something that was in contact with fecal matter, she said.

The types of food that can carry the disease include dairy products and chicken, McAllister said. Undercooking food or storing it at temperatures not low enough can allow the bacteria to multiply, McAllister said.

As many as 3 million salmonella cases are reported in the United States annually. The disease can be dangerous, even fatal, to children and elderly people and others whose immune systems are weak.

Restaurant personnel could not be reached for comment last night.

McAllister is asking anyone who has eaten at the Oxon Hill restaurant since May 30 and has symptoms of the disease to call Prince George's County health officials.