Various preparations of Staphylococci protein A (SpA) obtained either from strain Cowan-1 or strain A676 and two SpA low molecular fragments (AB and B) were tested for their mitogenic and NK-stimulating activity on human peripheral blood lymphocytes. Though all 4 preparations of SpA and both of its fragments were able to react with human IgG, apparently with both Fab and Fc sites of IgG, only the SpA preparations derived from A676 strain triggered the proliferation and potentiated the NK-activity of treated cells. The parallelism between the mitogenic and NK-stimulating activities of SpA suggests that both activities might be under the control of the same portion of the SpA molecule located in the non-immunoglobulin binding region of the molecule.