C-reactive protein (CRP) is an acute phase protein which shares with the immunoglobulins the ability to induce precipitation and agglutination reactions and activate the complement system. We report here that purified human CRP binds selectively to human T lymphocytes, inhibits their ability to form spontaneous rosettes with sheep erythrocytes and inhibits their response to allogeneic cells in mixed lymphocyte culture reactions; it fails to inhibit phytohemagglutinin- or concanavalin-A-induced mitogenesis. CRP does not bind to human B lymphocytes, nor does it alter the following B-cell functions: binding to activated complement components or the Fc portion of immunoglobulins, mediation of antibody-dependent cytotoxicity reactions or the ability of allogeneic cells to stimulate a mixed lymphocyte culture reaction. Human CRP shows similar selective binding with murine T lymphocytes. It therefore seems that binding of CRP is a property of T lymphocytes or a subpopulation thereof, and can result in modulation of certain of the T-cell functional characteristics in vitro. We suggest that CRP may play a role in modulating T-cell functions during the inflammatory state.