Studies were carried out on the ability of protein A (PA) and of muramic acid (MA) from S. aureus to induce the release of cytokines both from monocytes and lymphocytes in vitro. Results show that protein A induces the greatest activity, compared to the activity already known for the theicoic acid (TA) and for muramyl dipeptide (MDP). At concentration of 10 micrograms/ml; PA induces roughly +180% release of TNF with respect to controls, while release of IL-1 alpha is about 500% control values, and is higher than those obtained when cells are treated with TA and MDP; IL-6 release is higher than that stimulated by Con A, used as standard challenge. At PA concentrations of 5 micrograms/ml, IL-4 release is about five times higher than that induced by Con A. Release of IFN-gamma showed similar dose-dependent stimulations. Muramic acid (MA) is particularly active in inducing the release of cytokines from target cells, inducing TNF release of about +75% with respect to the controls. This increase is less than that obtained with PA. Also IL-4 and IFN-gamma are released by PA in quantities higher than those induced by TA and MDP. Our results lead us to believe that during infections by Gram-positive bacteria, their surface components are able to induce a series of chain reactions ranging from the inflammatory to the immunologic responses which are also conditioned by release of cytokines.