There is no evidence that the measles, mumps and rubella vaccine overloads children's immune systems or makes them more vulnerable to bacteria infections, researchers in Britain have found.

The researchers, from the British Public Health Laboratory Service,said they undertook their study because some British parents' groups contend that the M.M.R. vaccine gives children more viruses than they can cope with, weakening their immune systems. The findings were reported yesterday in The Archives of Disease in Childhood, a medical journal.

British and American groups skeptical about vaccinations said yesterday that the study, which was largely paid for by vaccine companies, missed the point. Their contention is that the vaccine overwhelms defenses against viruses, particularly measles, and makes infants vulnerable to autism.

Vaccine experts in the United States dismissed that idea as scientifically unsound and said the study's conclusions made sense, although some were disappointed at how little data the researchers released.

Fear of the M.M.R. vaccine in Britain is so great that immunization rates with it have dropped to 85 percent nationally since 1998, and much lower in some areas, despite strong government endorsement of immunization.

Even as he endorsed the M.M.R. vaccine, Prime Minister Tony Blair has refused to say whether his new son has had the shot, drawing loud criticism from editorialists. Small, local measles outbreaks across Britain have raised alarms.

The vaccine, given in the second year of life, is made with live, but weakened, viruses. The viruses provoke the body to produce antibodies, without actually causing disease.

The researchers looked at the medical records of all young children hospitalized in southern England for serious bacterial infections of the blood, brain stem and lungs between 1991 and 1995. It studied those who had had an M.M.R. inoculation in the previous three months and excluded those who had underlying immune system problems or had been hospitalized before.

Researchers concluded that immunized children actually had less risk than average of being hospitalized for pneumonia, meningitis, septicemia or other infections.

The results should be ''reassuring evidence for parents on the safety of M.M.R. vaccine,'' said Dr. Liz Miller, leader of the Public Health Laboratory Service immunization division and the lead author of the paper.

''While there were already strong scientific arguments against the idea that the immune system could be overloaded, our study provides direct evidence that this does not occur.''

Dr. Neal Halsey, director of the Institute for Vaccine Safety at the Johns Hopkins School of Public Health, called the study ''well done, with appropriate methods, by investigators who are well known.''

Groups skeptical of the M.M.R. vaccine on both sides of the Atlantic demurred. Dr. Marcel Kinsbroune, a pediatric neurologist who advises the National Vaccine Information Center, which considers some vaccines unsafe, conceded that the study showed that vaccinated children's immune systems do not collapse, but argued that ''the serious allegations against M.M.R. have nothing to do with bacterial infections -- they have to do with intestinal inflammation and autism.''

''The smoking gun,'' he said, ''is that measles vaccine is in the lining of your gut years later.''