An indirect solid-phase radioimmunoassay (RIA) was applied to titration of serum and cerebrospinal fluid (CSF) antibodies against a variety of viruses including rubella, mumps, measles, herpes simplex, varicella-zoster, and vaccinia. The test used fixed, virus-infected cells as a source of antigen, and conditions for optimal production of viral antigen were determined for each virus-host cell system. In acute, uncomplicated viral infections, sera taken 2 to 5 days after onset generally had low homotypic RIA titers ranging from less than 1:100 to 1:500, whereas convalescent-phase titers ranged from 1:128,000 to 1:512,000. Rubella and measles antibody titers as high as 1:256,000 were demonstrated by RIA in CSF from patients with chronic panencephalitis, whereas homologous antibody titers of 1:4,000 were detected in CSF from acute mumps, herpes simplex, and varicella-zoster virus infections with central nervous system involvement. Some heterotypic antibody was demonstrable by RIA in CSF, but, with the exception of herpes simplex antibody in a mumps virus infection, titers were markedly lower than those to the infecting virus type. RIA generally demonstrated titers at least 1,000 times higher than those obtained by conventional assays such as complement fixation, hemagglutination inhibition, neutralization, and immunofluorescent staining.