The possible role of microfilarial surface (cuticular) antigens in immuno-diagnosis of human filarial infections has been assessed using microfilariae (mf) of the cattle parasite Onchocerca gibsoni. A Triton X-100 extract of 125I-labeled O. gibsoni mf was reacted with a panel of sera from humans infected with Onchocerca volvulus, Wuchereria bancrofti and Schistosoma japonicum as well as sera from uninfected controls. Results of these immunoprecipitations indicated that sera from humans infected with O. volvulus or W bancrofti contain antibody specificities recognising certain of the radioiodinated cuticular proteins of O. gibsoni mf. Two-dimensional gel analysis and subsequent autoradiography of these immunoprecipitates showed that 8 radioiodinated proteins recognised by sera from calves injected with O. gibsoni mf were also immunoprecipitated by sera from humans infected with either O. volvulus or W. bancrofti. Thus there appear to be no major radioiodinated cuticular antigens of O. gibsoni mf which are species or onchocerca specific.