Soluble C3d applied to human blood-derived B lymphocytes inhibited anti-mu, T cell-produced growth factor, and EBV-induced DNA synthesis in serum-free culture. C3d added to the B cell cultures 1 and 2 days after the stimulus, still exerted inhibition, though with gradually diminishing efficiency. C3d, fixed on zymosan or attached to the culture wells, induced [3H]thymidine incorporation of the B cells in serum-free medium. The concentration of C3d used to coat the wells was critical, with optimal stimulatory effect of 8.3 micrograms/ml. These C3d molecules were shown to be denatured. Our results are in line with earlier data on B cells derived from mouse spleen and human tonsils showing that depending on the way of presentation and its amounts, the natural ligand of CR2 can exert negative or positive signals. Moreover, we demonstrate that C3d can inhibit even the proliferative stimulus of EBV.