1) Additional inhibitor studies on the aspartyl proteinase from Rhizopus chinensis have been carried out. The inhibitors, analogs of pepstatin, provide information regarding the mechanism of action of this class of enzyme and support the concept of rational drug design based on a knowledge of three-dimensional structure. 2) The structure of the enzyme tryptophan synthase from Salmonella typhimurium has been further studied and partially refined. The enzyme, which is bifunctional with an alpha and beta reaction, is organized in an approximately linear alpha beta beta alpha structure, about 145 A long. The most striking result is the demonstration of the presence of a tunnel linking the two active sites, thus permitting the diffusion of indole, the product of the alpha reaction, to the active site of the beta enzyme.