An anti-Gc antiserum (anti-Gc) was prepared by immunizing rabbits with purified Gc, a major vitamin D-binding protein in human serum. Gc is expressed on the surfaces of a small proportion of resting T cells and almost all B cells are natural killer (NK) cells. We investigated the effects of anti-Gc on NK activity of normal peripheral blood lymphocytes. Both anti-Gc and purified IgG isolated from anti-Gc prominently inhibited human NK activity in vitro. Inhibition of anti-Gc was dependent on the concentration of the antiserum employed. Competition assay showed that purified Gc significantly blocked the inhibitory effect of anti-Gc. The inhibition of NK activity in the cells treated with anti-Gc was restored time-dependently by being cultured in anti-Gc free medium. Anti-Gc-treated NK cells retained their capacity to bind to target cells. These findings suggest that mGc is associated with the process of the NK cytolysis on the post-binding cytolytic phase.