A new antibiotic was obtained from the culture broth of Streptomyces alanosinicus 879-MT3, and the name spicamycin was given. Spicamycin had a marked effect on the induction of differentiation of human promyelocytic leukemia cells (HL-60) as well as mouse myeloid leukemia cells (M1). Its structure was elucidated by degradative studies and 1H and 13C NMR spectral analysis as shown in Fig. 1. During the course of our screening program for new differentiation inducers of myeloid leukemic cells, we isolated an active substance from the culture broth of a streptomycete, and the name spicamycin was given. It can induce differentiation of M1 cells and HL-60 cells and shows antitumor activities. This paper describes the isolation, structural determination and biological properties of spicamycin, as well as the taxonomy of its producing organism Streptomyces alanosinicus 879-MT3.