We analysed the reactivity of enterovirus-specific human IgM and IgG antibodies with the structural proteins of different enteroviruses by the immunoblot technique. In general, all immunoglobulin G antibodies of the tested sera reacted with capsid polypeptide VP 1 of the viruses tested (echoviruses 9 and 11, coxsackievirus B3 and poliovirus 2). In contrast, enterovirus specific immunoglobulin M antibodies of adults reacted with capsid polypeptides VP 1, VP 2, and/or VP 3 of the viruses mentioned above. The reactions with VP 2 and/or VP 3 were often stronger than with VP 1. IgM antibodies from sera of newborns infected by echovirus 11 reacted with VP 1 and VP 2/3 of echovirus 11 and also with VP 2 and VP 3 of poliovirus 2. Preabsorption experiments indicate that cross-reactive IgG antibodies react with epitopes of VP 1 not present on the surface of intact virus particles. The results from the immunoblot technique were compared to data from microneutralization tests and M-antibody capture radioimmunoassays.