A subpopulation of human lymphocytes (CR+ lymphocytes) possesses receptors for the third component of complement (C3). This paper examines the rate of incorporation of uridine in RNA of CR+ lymphocytes compared to lymphocytes without complement receptors (CR- lymphocytes) and the effect of interaction of lymphocytes with human complement-coated autologous red cells (EC). CR+ lymphocytes had a greater baseline RNA synthesis than CR- lymphocytes. Stimulation of RNA synthesis was observed in CR+ lymphocytes after 3 hr of incubation with EC, as compared to lymphocytes incubated with red cells which lacked membrane-bound complement (E). RNA synthesis in CR- lymphocytes was unaffected by incubation with EC. This increased uridine incorporation into RNA in CR+ lymphocytes was due neither to increased intracellular flux of uridine nor to decreased degradation of RNA. Lymphocytes incubated with EC had increased incorporation of amino acids into intracellular proteins as compared to controls. Thus, the interaction of complement-coated red cells with complement-receptor-bearing human lymphocytes causes an immediate increase in lymphocyte RNA synthesis. © 1977.