The interaction of Ce with immunologically competent cells (lymphocytes and macrophages) is currently under investigation. Upon immune activation or controlled proteolytic digestion, C3 is cleaved into two fragments, C3a, and C3b. The larger of these, C3b, binds to specific receptors on both lymphocytes and macrophages. Present studies are concerned with the activation of lymphocytes by this fragment and the conditions which influence the macrophage C3b receptor. Lymphocytes are stimulated to produce a chemotactic lymphokine in response to C3b. Studies concerning the expression of C3b receptors on macrophages demonstrate marked dependence both on the method by which the cells are obtained and the time they are maintained in culture. The functional significance of the alterations in C3 receptors on macrophages is presently being examined. The C3b fragment is further cleaved in vivo to two additional fragments, C3c and C3d. The C3d fragment which can also be generated in vitro by proteolytic cleavage, also interacts with a specific cell receptor and its possible modulatory role in cellular immune function is under investigation. BIBLIOGRAPHIC REFERENCES: Koopman, W. J., Sandberg, A. L., Walh, S. M. and Mergenhagen, S. E. Interaction of soluble C3 fragments with guinea pig lymphocytes, comparison of effects of C3a, C3b, C3c and C3d on lymphokine production and lymphocyte proliferation. J. Immunol. 117: 331-336, 1976.