A novel macrolide, isolated from the myxobacteria Sorangium cellulosum WXNXJ-C, was identified as 1,7,12,13-tetrahydroxy-14-methoxy-8,10-dimethyl-6-phenyl-5,15-dioxa-bicy clo[9.3.1]pentadecan-4-one, "Phoxalone". It had minimum IC50 values of 0.24, 6.9, 10.3, 0.98 and 4 mu g/ml, respectively, against tumour cell lines: B16, Bel7402, H446, MCF-7, and SGC7901. In addition, it had less cytotoxicity to normal human liver L02 cell lines (286 mu g/ml, 24 h). A cytotoxic bioactivity study on H446 cell line in vitro suggested that Phoxalone arrested the mitosis in the G2/M phase.