We have developed a highly specific and sensitive "two-site" immunoradiometric assay for murine IgM in which antigen bound to immobilized antibody reacts with affinity-purified radiolabeled antibody. We utilized the sensitivity of this assay to study the rate of IgM secretion in short-term cultures by spleen cells from the autoimmune strains, New Zealand Black (NZB) and New Zealand Black by New Zealand White F1 hybrid (BW), and from normal (C57BL/6, DBA/2, NZW) mice. The temperature dependence of IgM secretion in short-term cultures, its pentameric structure, and the similar viability of NZB and normal strain spleen cells indicate that active IgM synthesis is being measured. We observed that the splenic B lymphocytes of NZB and BW mice, in contrast to normal strains, produce IgM in vitro at birth. By 6 to 8 weeks of age NZB and BW spleen cells produce 40 times more IgM than spleen cells from normal strains of mice. The IgM produced in vitro by splenic lymphocytes from NZB and BW mice is not absorbed by synegeneic or allogeneic thymocytes or erythrocytes.