Improved methods for diagnosis of onchocerciasis are needed. We have recently identified immune complex-associated parasite antigens in sera from onchocerciasis patients. The goal of this study was to produce monoclonal antibodies to these antigens that might be used in antigen detection assays. Two monoclonal antibodies (OV-1 and OV-5) that bind to parasite antigens in immunoblots of PEG-precipitated immune complexes from human onchocerciasis sera and to corresponding antigens in adult worm extracts and excretory-secretory products were produced. The target epitopes of the monoclonals are heat stable, resistant to trypsin, and destroyed by Pronase. The two monoclonals produce similar but not identical patterns of binding to immunoblots of Onchocerca volvulus adult worm antigen with major bands at 43-47, 58-63, and 70 kDa. OV-1 and OV-5 appear to bind to two distinct but closely related epitopes, neither of which is phosphorylcholine. Immunoelectron microscopy showed that the epitopes recognized by these monoclonals are widely distributed in adult female worms, but concentrated in the uterus and intestine. Antigen assays based on these antibodies detected parasite antigen in 9 of 14 sera from onchocerciasis patients, but significant background signnal was detected in some nonendemic human sera. Thus, although this study has provided new information on parasite antigens in sera from onchocerciasis patients, additional work will be needed to achieve the goal of producing a sensitive and specific antigen diagnostic test for onchocerciasis.