Previous studies in experimental animals have documented that immune complexes consisting of antigen and antibody exert a regulatory effect in immunological response. Using peripheral blood human lymphocutes from subjects previously immunized with Keyhole Limpet Hemocyanin (KLH), the present study compared the in vitro lymphocyte blastogenesis induced by KLH with that mediated by precipitated immune complexes formed by KLH and IgG anti KLH. The IgG antibody complexed to the antigen consistenly induced a proliferative response significantly higher than that mediated by the antigen alone. The enhancement observed with the immune complexes was neither dependent of the presence of complement or the amount of antibody molecules in the complexes. Depletion of non T, non B lymphocytes with high avidity Fc receptors for molecules of IgG, reduced the blastogenesis response induced by immune complexes to levels observed with the antigen alone. We conclude that these lymphocytes played an important role in the enhancement of lymphocyte blastogenesis mediated by immune complexes, probably by their ability to bind the IgG through Fc receptors.