Onchocerca volvulus is a nematode that causes severe dermatitis and blindness in humans. Prior studies have shown that immune complexes in sera from onchocerciasis patients contain parasite antigens with Mr of 23,000 and 65,000-70,000. Monoclonal antibody OV-1 binds to these antigens and to corresponding antigens in adult worm extracts and in vitro culture supernatants. OV-1 was used to immunoscreen an O. volvulus adult worm cDNA library. Clone OV1CF contains a 1632-bp open-reading frame that codes for a protein with a predicted Mr of 63,000. The deduced protein sequence of OV1CF has 78% identity with Ascaris lumbricoides intermediate filament A and 40%-50% identity with several mammalian intermediate filaments. Antibodies raised to OV1CF bind to human keratin, and some sera from onchocerciasis patients contain antibodies to OV1CF and to keratin. Thus, immune complexes from onchocerciasis patients contain a parasite antigen that is immunologically cross-reactive with human intermediate filament proteins.