Most studies on immunologic responses to Onchocerca volvulus have employed extracts or antigens from related filarial parasites. Consequently, little is known about the nature of O. volvulus antigens. Potential antigen sources include in vitro cultures and physicochemical fractionation of O. volvulus extracts. IgE antibody responses to a wide range of antigens occur in patients with onchocerciasis, but there is little evidence of species specificity in serologic tests. Some potent allergens are released by microfilariae, but host serum proteins appear to contaminate the most reactive fractions obtained thus far. Antibody-mediated cell adherence to microfilariae of O. volvulus occurs in vitro, and both stage and species specificity have been demonstrated. Monoclonal antibodies to O. volvulus antigens have been prepared, but, unfortunately, all to date show cross-reactivity to antigens of other filarial nematodes. Circulating antigens have been detected in patients' sera; no data are available yet on the specificity of these components. Research needs include the need for species- and stage-specific reagents for immunodiagnostic assays and for investigations on immunopathogenic mechanisms in onchocercal disease.