The antigenic specificity of antibody-dependent plaque enhancement (ADPE) on the macrophage cell line P388D1 has been investigated using viruses in the flavivirus and alphavirus genera. Whereas ADPE detected group-specific antigenic determinants in the flaviviruses, narrower specificity (type and subgroup) was seen with alphaviruses. ADPE titres correlated well with haemagglutination inhibition titres.