Monoclonal antibodies, 3B9 and 4C9, specific to connectin (also called titin), 3000 kDa elastic filamentous protein of vertebrate skeletal muscle, crossreacted with a high molecular weight protein (500 kDa) of the nematode Caenorhabditis elegans. However, its crossreactivity was weak to that of the unc-22 gene deficient mutant. Immunofluorescence showed that the antibodies stained both bodywall and pharynx muscles in the wild type, but only pharynx muscle in the mutant. Immunoelectron microscopy revealed that the antibodies bound to the dense bodies of bodywall muscle cells of the wild type but not to those of the mutants. In the pharynx muscles the localization of the antibodies was not clear in both normal and mutant worms. Moerman, D.G. et al. (Genes & Development 2, 93-105 (1988) reported that the unc-22 gene product (500 kDa) is located in the A band of the bodywall muscle cells of C. elegans. Taking this information into consideration, it is suggested that the unc-22 gene product may be a qfilamentous protein linking a dense body and myosin filaments in the bodywall muscles of C. elegans.