Funding is requested for an integrated state-of-the-art x-ray structure determination and analysis system possessing: a computer controlled diffractometer with a high intensity rotating anode x-ray source; capabilities for collection of x-ray data at low temperature; a fast multi- user microcomputer with a large disk for storing and manipulating structural data bases. This research equipment will be used for several diverse projects. These include: structural studies of organo sulfur compounds and organo selenium compounds; structural studies of synthetic models for the metal centers of molybdenum-containing enzymes; low- temperature structural studies of unsaturated fatty acids and asymmetric (mixed-chain) phospholipids; structural studies of conformationally constrained peptides and of hormones, for understanding information transfer by biological messengers, for comparison of solid state and solution structures, and for designing new compounds with superior biological and medicinal properties; structural studies of natural products, antitumor agents, and antibiotics; modeling the interactions between drugs and macromolecules, especially the binding of antitumor agents to DNA; correlation of specific molecular distortions with chemical and biological activity; high resolution experimental electron density studies of organo sulfur compounds, metal-sulfur bonds, and models for molybdenum enzymes.