IgM antibody capture radioimmunoassays were developed to detect coxsackie virus B1-B5 specific IgM. Specific IgM was detected in sera from all patients with coxsackie B virus infections proved by isolation; however, sera from 13/32 patients with rising neutralising antibody titres were negative in the assay. Frequent heterotypic responses were seen among the positive sera. Thirty seven patients with other enterovirus infections were also studied, and sera from 15 of these patients reacted in the assay, showing that heterotypic coxsackie B IgM responses occur not only in coxsackie B virus infections but also in other enterovirus infections.