Of 78 individuals subjected to intimate exposure to mumps whose tests were originally negative and who failed to develop the disease, 41 per cent gave positive reactions when tested 1 month later.

Seventy-seven per cent of complement fixation tests done on the sera of 565 normal adults who admitted a previous attack of mumps were positive. A similar correlation was recorded in tests on the sera of a small group of children with positive histories. Of 356 medical students admitting previous attacks, 80 per cent gave positive tests. Of 386 normal adults who denied previous attacks, 42 per cent gave positive tests; of 85 children giving negative histories, 38 per cent reacted positively.

The results of complement fixation tests on the sera of 1665 normal adults (over 17 years) and 679 children (1 to 17 years) are recorded. It has been shown that 63 per cent of the adults and 57 per cent of the children had antibody in their blood which reacted with the virus of mumps. In groups in which exceptionally intense exposure was not known to have occurred in the past, the proportions of positive reactors were: adults, 61 per cent; children, 49 per cent. In contrast to these normal persons, the incidence of positive reactors among permanently institutionalized mental defectives was 38 per cent of 356 adults and 32 per cent of 475 children.

In only 2 per cent of 320 normal adults and children did the titer of complement-fixing antibody reach 1–192. In no instance in which the endpoint was determined was a higher titer recorded.

The results of complement fixation tests on the sera of mother and newborn infant were essentially the same in 5 instances.