The ternary complex of the Bowman-Birk inhibitor with alpha-chymotrypsin and beta-trypsin has been crystallized and preliminary crystallographic information describing the crystals has been obtained. The crystals are triclinic with unit cell dimensions: a = 51.96 A, b = 56.34 A, c = 46.70 A, alpha = 98.83 degrees, beta = 97.29 degrees, and gamma = 97.15 degrees. There is 1 ternary complex/unit cell and the diffraction pattern extends to 2.2 A resolution. The search for heavy atom derivatives based on known derivatives of chymotrypsin and trypsin is underway.