The immune state was evaluated over a 10-year period in two individuals with Bloom's syndrome. In both patients, serum concentrations of IgM were markedly low. Mildly decreased serum concentrations of IgG and IgA increased significantly with age, whereas the IgM levels remained low. From assessments of B-cell and T-cell functions in pokeweed mitogen-induced immunoglobulin production, the IgM deficiencies were thought to result from B-cell dysfunction. T-cell function appeared intact. Moreover, although the percentages of surface IgM-bearing cells were not reduced, the numbers of IgM-secreting cells were reduced. These findings suggest that the IgM deficiency is due to an abnormality in the maturation of surface IgM-bearing B cells into IgM-secreting cells.