Antibodies of the IgM class are elaborated by many common murine hybridoma cell lines and often define important antigens. In contrast to IgG antibodies, however, IgM molecules exhibit little or no affinity for bacterial Ig binding proteins, such as protein A and protein G, which are almost universally used to detect and/or bind IgG antibodies in cytofluorography, Western transfer, immunoprecipitation, and so forth. It is therefore highly desirable, in laboratories in which hybridoma technology is routine, to have in place a means of detecting and purifying IgM molecules, sometimes on a modest or large scale. The most efficacious and general method for IgM detection and purification still involves the appropriate use of heterospecific antimurine IgM antibodies.