In a prospective 2 year study of 59 cases of childhood meningitis, mumps was the most common etiological virus (39%), followed by enterovirus (27%). The analysis of the cases suggested that a diagnosis of the infectious agent may be arrived at using clinical data such as the degree of nuchal rigidity, the age of the patients, and the presence of associated parotiditis or macular rash. Pleiocytosis in the CSF was higher and included a larger percentage of lymphocytes during mumps meningitis than during enterovirus meningitis. Mumps or enterovirus were isolated in the CSF of 23% (mumps) and 27% (enterovirus) of the patients. Alpha interferon which was acid labile was detected in the CSF of 89% and 63% respectively of patients with mumps and enterovirus meningitis.