The existence of a leukocyte complement system is suggested by the findings that certain lymphocytes and mononuclear phagocytes activate complement, express membrane receptors specific for complement activation fragments, and synthesize complement proteins. In this review, attention is directed toward studies indicating that complement proteins may be expressed on the surface of lymphocytes and monocytes, the mechanisms by which complement is activated by leukocytes, and the possible role(s) of this cellular complement system in processes by which lymphocytes are stimulated and monocytes are activated to increased cellular activity.