Recent data indicate that heparin may act as an immunomodulator. In this paper we have analyzed the effects of this agent on human B cell proliferation in vitro induced by S. aureus Cowan. The action of heparin is complex, but there was a trend for inhibition of B cell responses obtained from defibrinated but not heparinized blood samples. This suggests that heparin interacts with platelet products (growth factors, cytokines) and the results of such interactions determine the final effect.