Thiocillins I and II were isolated from the culture broth of Bacillus cereus G-15, and thiocillins II and III from that of Bacillus badius AR-91. Also, the former two were probably produced by Bacillus megatherium I-13. These antibiotics active against Gram-positive bacteria are soluble in a mixture of chloroform and methanol, show characteristic ultraviolet absorptions (maxima at ca. 275 nm and ca. 348 nm), and contain a high content of sulfur, as much as approximately 15%. They are related to each other and also to micrococcin P, but differentiated by chromatographic behaviours.