Several inhibitors of fatty acid synthesis have recently been noted as a new group of antibiotics which show selective toxicity against procaryotes. Thus, thiolactomycin selectively inhibits the fatty acid synthetase of Escherichia coli (type II) but has little effect on that of mammalian tissues (type I). On the other hand, cerulenin inhibits both type I and type II synthetases. In order to find new inhibitors of the fatty acid synthetase of E. coli (type II), we employed a fatty acid synthetase assay system comprising fraction A and the acyl carrier protein (ACP), which was prepared by essentially the same method as described by MAJERUS et al. from E. coli K12 except that the ACP preparation was used without further purification by DEAE-cellulose and DEAE-Sephadex chromatography. Three active compounds were isolated from the culture filtrate of Streptomyces strain No. S-1998. These antibiotics belonging to the ansamycin group were named naphthoquinomycins A, B and C.