One of the products of the metabolism of Streptomyces mediterranei is rifamycin Y, a compound chemically and physically similar to the antibiotic rifamycin B, C28H40NO14, and yet biologically inactive. Apart from this qualitative information, little was known at the outset about the structure of rifamycin Y. No explanation was therefore possible for the difference in the activity of the two compounds and for the even vaster difference in the activity of their respective derivatives. In order to determine the constitution and the complete three-dimensional structure of rifamycin Y, and to compare it with that of rifamycin B, which had already been studied both by chemical and crystallographic methods, the crystal structure of the p-iodoanilide derivative of rifamycin Y was elucidated in our laboratory. The constitution and, in part, the configuration of rifamycin Y was also determined by LEITICH, PRELOG and SENSI by chemical and spectroscopic methods. We are now very pleased to find that the independent results of the 2 groups agree completely.