Antiserum was solid phase adsorbed to ensure class specificity. Antiserum was cross adsorbed using mouse and rat immunosorbents to remove cross reactive antibodies. By immunoelectrophoresis and ELISA this antibody reacts specifically with human IgM. Cross reactivity with IgA and IgG is negligible. No antibody was detected against non-immunoglobulin serum proteins. Less than 1% cross reactivity to mouse and rat IgM was detected. This antibody may cross react with IgM from other species.