C5a derived from the fifth component of human complement was found to potentiate both specific and non-specific human in vitro humoral immune responses. In addition, C5a is capable of potentiating antigen- and alloantigen-induced T cell proliferative responses. In contrast to these results, C5a was unable to modulate mitogen-induced B and T cell proliferative responses. The carboxyterminal arginine of C5a is not essential in order for C5a to enhance immune responses. C5ades Arg was found to augment the immune response to the level of C5a-mediated enhancement. The serum carboxypeptidase inhibitor, 2-mercaptomethyl-5-quanodinopentanoic acid, which prevents cleavage of the terminal arginine, allowed us to assay the effects of C5a on in vitro immune responses in the presence of serum. Helper T cells are involved in C5a-mediated immuno-potentiation. Substitution of T cells by soluble T cell-replacing factors, (Fc)TRF, rendered lymphocyte cultures refractory to the enhancing properties of C5a.