Y7, a murine monoclonal IgG1 kappa antibody against a human monoclonal IgM lambda DJ molecule, was affinity purified on an IgM lambda immunoaffinity column. As detected by enzyme-linked immunosorbent assay (ELISA) the isolated Y7 monoclonal antibody was shown to be not cross-reactive with human IgG, human secretory IgA, mu chain, lambda + kappa chains and another human monoclonal IgM lambda BR. Binding to the polyclonal human IgM standard in the same assay was about 30 percent. The epitope specificity of affinity purified and biotinylated Y7 MoAb was localized only in the nonreduced pepsin Fab fragments of IgM lambda DJ immunogen. As the immunogen was determined to be a specific antibody to phosphorylcholine, the specificity of Y7 MoAb was further ascertained in its capacity to induce 95% inhibition of immunogen binding for phosphorylcholine.