The cross-reactivity of 22 arbovirus species (alphaviruses; tick-and mosquito-borne flaviviruses and selected bunyaviruses) was tested with, monovalent immune mouse ascitic fluid by indirect immuofluorecence (IIF) in comparison with cIasssical serologic reaction (virus neutralization (VN); hemagglutination inhibition (HI); and complement fixation (CF)). Knowm relationship within the virus groups studies were confirmed. As to differenciation limits, the VN test was followed by IIF. Evaluation of the ratio of heterologous to homologous antibody activities showed that the exception of antigenically closely related arboviruses (Western equine encephalomyelitis, Sindbis, Japanese encephalitis, Murray Valley encephalitis, dengue viruses, California, Tahyna), most arboviruses within the antigenic complexes tested could be relatively reliably differentiated by IIF.