Several fragments derived from complement components have been identified as potent effector substances in in vitro assays that measure cell proliferation and antibody synthesis. The anaphylatoxin C3a suppresses the immune response but fails to influence T- or B-cell proliferation. The factor C5a augments both antibody production and antigen-induced, but not mitogen-induced, T-cell proliferation. C3a-mediated suppression occurs through the activation of a suppressor T-cell cascade with macrophage collaboration. C5a-mediated enhancement, depending upon the in vitro system studied, acts at the level of the helper T cell and/or macrophage. A fragment generated from treating iC3b with kallikrein (c3d-K) has aided in defining a structural region of the C3b molecule that can influence the level of circulating leukocytes. The factor C3d-K is also capable of suppressing both specific and non-specific T-cell proliferative responses and mitogen-induced B cell growth. The mechanism of C3d-K action is defined as a direct effect on "activated" T cells, even though IL-2 synthesis of treated cells is diminished. The effect of C3d-K is long lasting, non-reversible and requires only a short exposure to the target cell.