Adults of Onchocerca volvulus and Onchocerca gibsoni were identically fractionated into a surface-enriched fraction, a phosphate buffered saline (PBS) extract and a PBS insoluble-detergent soluble fraction. Glycoproteins were prepared from these extracts and all fractions were examined by the Western blot technique using sera from individuals infected with a variety of filarial and non-filarial nematode worms. Using antisera to O. volvulus, a number of antigens were demonstrated in all of the extracts, with some antigens of each extract being unique. Many antigens were glycoproteins, and a high cross-reactivity was observed between O. volvulus and O. gibsoni. The different fractions of both species were also analysed using a panel of different sera in order to identify Onchocerca-specific antigens. The studies revealed that the lower molecular weight antigens showed greater Onchocerca specificity in all of the extracts examined. The surface-enriched fraction, however, clearly contained less widely cross-reacting components than the somatic and glycoprotein fractions. Finally, using surface labelling and coprecipitation techniques, O. gibsoni was shown to possess a 20 kDa Onchocerca-specific antigen, previously described for O. volvulus. The findings indicate a number of Onchocerca-specific antigens which may have potential in diagnosis of human onchocerciasis. They also show that the related bovine parasite O. gibsoni, may be an alternative source of material.