Cytoadherence and cytotoxicity reactions against larval stages of Onchocerca volvulus may be protective or pathogenic. To elucidate the inherent mechanisms, we examined a panel of human, rabbit and mouse antisera to O. volvulus antigens. Human IgG, but not IgM from onchocerciasis sera promoted the adherence of leucocytes to microfilariae (mf) that resulted in the killing of the mf in the presence of heat labile serum factors. Polyclonal and monoclonal antibodies to three recombinant antigens of O. volvulus (OV7I, OV25 and O13) were assayed for their abilities to induce cellular adherence and/or cytotoxicity of nodular mf. These antibodies were found to promote cytoadherence but showed negligible cytotoxicity effects. We conclude that IgG antibodies from onchocerciasis sera are implicated in the adherence to and killing of mf by patients' leucocytes.