1. Fifty-one patients exhibiting signs and symptoms compatible with the clinical syndrome of acute aseptic meningoencephalitis have been studied by means of the complement fixation test for the demonstration of antibody reacting with the virus of mumps or its products.
2. In 17 of these individuals in whom the clinical diagnosis could be made with assurance because of an associated enlargement of the salivary glands, the test yielded evidence of recent infection with the virus.
3. In 16 patients without involvement of the salivary glands the results of the test likewise indicated recent infection with this virus.
4. In the remaining 18 patients the test gave no evidence of a recent infection with the virus.
5. Except for histories of recent exposure obtained from certain of the individuals in whom the complement fixation test was considered to be positive, the characteristics of the disease and the data afforded by the usual laboratory studies failed to distinguish the last 2 groups.
6. On the basis of these findings, it is concluded that the complement fixation test should be of value in the diagnosis of those cases of acute aseptic meningoencephalitis in which the virus of mumps is the etiologic agent and in which involvement of the salivary glands is slight or absent.