Antigens in Onchocerca volvulus were characterized using anti-O. vovulus monoclonal antibodies (McAb). A total of 17 McAb which were not species-specific to O. volvulus was examined for the antigenic similarity of O. volvulus with 12 parasitic helminths using indirect immunofluorescence. The range of reactivity of these McAb was wider in nematodes in contrast to species of cestoda or trematoda. The antigenic similarity within helminths examined was generally in agreement with their taxonomic relationships but was not shown in the lower taxonomic hierarchy. The helminth which holds the antigens most in common to O. volvulus was not the filarial parasites but Ascaris suum, an intestinal nematoda. The least antigenic similarity to O. volvulus was found in Schistosoma mansoni and S. japonicum, trematoda, among helminths examined. On the other hand, the existence of a common antigen shared with all the 13 helminths examined was demonstrated by a IgM McAb.