The biosynthesis of three complex antibiotics of microbial origin will be studied: the sesquiterpene pentalenolactone, the broad spectrum macrolide antibiotic erythromycin A, and the anticoccidial polyether antibiotic monensin. Each of the substances can be considered a prototype of a larger class of related natural products. A detailed examination of the biosynthesis of the three individual metabolites will allow a test of stereochemical models which are advanced to explain the formation of each class of substances. Pentalenolactone is one of a number of humulene-derived sesquiterpenes. Both 13C and 2H nmr will be used to establish the stereochemistry of the transformations by which humulene is formed and further cyclized. A theory to explain the biochemical basis of the intriguing structural and stereochemical homology among the more than fifty known macrolides will be tested by determining the origins of the oxygen atoms of erythromycin A. This theory is then extended to the polyether natural products, and experiments to establish the origin of several of the oxygen atoms of monensin are described. The goals are to 1) test a stereochemical model for the formation of humulene derived metabolites; 2) test a stereochemical model for macrolide biosyntheses and examine its applicability to the formation of polyether antibiotics; 3) probe and extend the utility of carbon and deuterium magnetic resonance studies in biosynthetic investigations, including the use of heteronuclear double labeling.