Mitogenesis and polyclonal immunoglobulin production in peripheral blood lymphocyte cultures activated with Formalin-fixed or autoclaved protein A-containing Staphylococcus aureus were studied. Direct evidence for a dissociation between cell proliferation and polyclonal immunoglobulin production was found, in that S. aureus was not mitogenic after being autoclaved but retained the ability to stimulate B cells to produce immunoglobulin. Trypsin-treated S. aureus lost its binding site for immunoglobulin G, but its mitogenicity was not altered; thus, the protein A binding site for immunoglobulin G on the bacterial cell wall is not required for the stimulation of lymphocyte proliferation. Our data also show a dissociation between cell proliferation and polyclonal immunoglobulin production induced by protein A coupled to Sepharose CL-4B. These results suggest the presence of three distinct active sites on the protein A molecule: one that binds immunoglobulin G molecules, one that stimulates cell proliferation, and one that stimulates polyclonal immunoglobulin production.