Immune function was studied in four patients (two girls and two boys, aged 30 months to 24 years) with documented Bloom's syndrome. Three patients had a decreased serum concentration of at least one subclass of immunoglobulins. All had normal or elevated proportions of circulating B cells but two of them had a decreased proportion of CD4-positive helper-inducer T cells. We consistently found a severely impaired in vitro proliferative lymphocyte response to the plant lectin pokeweed mitogen (PWM). This could not be overcome by using suboptimal or supraoptimal doses of PWM, or by adding recombinant interleukin 2. In vitro PWM-induced IgM production was absent or low in two of the three patients studied and this low production could not be increased by addition of hydrocortisone. T lymphocytes responded normally to the plant lectins phytohemagglutinin and concanavalin A. T cells preactivated with phytohemagglutinin also normally proliferated in response to interleukin 2. It has previously been shown that lymphocyte activation with PWM involves both B and T cells and proceeds via an alternative pathway. The data thus indicate that patients with Bloom's syndrome have a specific defect in this PWM-induced alternative pathway of lymphocyte activation.