Human blood lymphocytes were stimulated in vitro by four Staphylococcus aureus strains. Activation of immunoglobulin-secreting cells was determined by a reverse plaque forming cell (PFC) assay, and proliferation by quantitation of thymidine incorporation. Whole killed S. aureus were slightly more efficient than water-soluble preparations in the form of sonicated extracts and culture supernatants. Two S. aureus strains rich in protein A (Cowan I and E 2371) and one S. aureus strain deficient in protein A (E 1369) were potent B-lymphocyte stimulators inducing maximal activity on day 6 of culture. Another S. aureus strain deficient in protein A (Wood 46) did not possess++ human lymphocyte stimulating capacity.