After the repeated injection of sea urchin [Lytechinus pictus] sperm guanylate cyclase into rabbits, antibodies to the enzyme were formed. These antibodies inhibited the particulate or the Triton-dispersed forms of the sperm enzyme by greater than 97%. The sperm adenylate cyclase, cyclic GMP phosphodiesterase, adenosine, triphosphatase, guanosine triphosphatase and 5''-nucleotidase enzymes were not affected by the antiserum. The antiserum inhibited the Triton-dispersed guanylate cyclase from rat heart, liver, lung, spleen and kidney but did not inhibit the soluble form of the enzyme from any of these tissues. The inhibition of the Triton-dispersed enzyme in these tissues was partial, however, ranging from 30% (liver) to 70% (heart). These results provide evidence that adenylate cyclase is antigenically different from guanylate cyclase, and that the soluble form of guanylate cyclase is antigenically different from a particulate form of the enzyme in various rat tissues.