Structures of chicamycins A and B have been determined from a series of chemical degradation studies coupled with spectroscopic analysis. Chicamycin A is 2(S),11(R), 11a (S)-1,2,3,10, 11, 11a -hexahydro-2,8-dihydroxy-7,11-dimethoxy-5H-pyrrolo-[2,1-c] [1,4]-benzodiazepin-5-one, and chicamycin B is 2(S), 11a (S)-1,2,3, 11a -tetrahydro-2,8-dihydroxy-7 -methoxy-5H-pyrrolo-[2,1-c][1,4]-benzodiazepin-5-one which is the demethanol form of chicamycin A. The structure of chicamycin B is closely related to that of neothramycin , differing only in the position of a hydroxyl substituent on the pyrrolidine ring.