The solid-phase radioimmunoassay (RIA) method developed in our laboratory for demonstrating rubella virus-specific immunoglobulin G (IgG) antibodies (Kalimo et al., 1976) was further developed for demonstrating IgM antibodies. A total of 188 serum specimens were tested. The statistical probability of obtaining a false-positive IgM result, based on determinations of 100 rubella-negative sera, was below 0.001. Nonspecific inhibitors and IgM antibodies against other viruses tested did not interfere in the assay. In 2 out of 20 (10%) serum specimens with rheumatoid factor, a false-positive IgM result was obtained. The new RIA method was compared with sucrose density gradient centrifugation, followed by hemagglutination inhibition testing of the separated immunoglobulins with respect to demonstrating IgM antibodies. In patients with acute rubella infection, IgM antibodies were demonstrated by RIA in 9 out of 20 acute-phase sera and in all 20 early-convalescent-phase sera, compared with 7 out of 20 acute-phase sera and 19 out of 20 early-convalescent-phase sera by sucrose density gradient centrifugation. The results obtained indicate that the RIA method is reliable and sensitive and suitable for routine diagnostic use
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