The release of histamine from rat mast cells induced by anti-immunoglobulin E (IgE) was markedly inhibited by Bowman-Birk soybean protease inhibitor (BBI) and anti-chymase F(ab')2 fragments which inhibit chymase activity [Kido, H., et al. (1985) Biochem. Int. 10, 863-871, and Fukusen, et al. (1987) Biochem. Med. Metab. Biol. 38, 165-169]. When radioiodinated anti-chymase F(ab')2 fragments or BBI were incubated with mast cells at 37 degrees C, they were subsequently recovered in the fractions of mast cell granules and of plasma membranes by Percoll density gradient centrifugation. However, when they were incubated with mast cells at 0 degrees C, they had no effect on histamine release and were subsequently recovered only in the plasma membrane fraction. Histamine release from mast cells was suppressed dose-dependently by the antibodies or BBI accumulated in mast cell granules at 37 degrees C. These results suggest that the antibodies and BBI are incorporated into mast cell granules and inhibit chymase activity, resulting in inhibition of histamine release.