Thiocillins I, II and III were compared with micrococcin P1 by analysis of acid hydrolyzates of the native and the reduced antibiotics as well as by means of 1H and 13CNMR spectroscopies. As a result of these studies, the differences of these antibiotics were clarified in their structural units, and the structures of thiocillins I, II and III were assigned on the basis of the proposed structure of micrococcin P1.