The proliferation of human B cells was studied for response to interleukin 2 (IL-2) produced in Escherichia coli using recombinant DNA technology. The IL-2 was found to be an homogenous preparation by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting using the anti-IL-2 monoclonal antibody DMS-1. IL-2 was found to stimulate B cell proliferation. Activation of the B cells using anti-IgM antibodies increased this response. Resting T cells from the same donors were found to be less reactive to IL-2. The results suggest that human B cell proliferation can be stimulated by IL-2 alone.