During the course of screening for new antitumor antibiotics, a triene lactone antibiotic IM8443T was found in the culture broth of Streptomyces hygroscopicus IM8443T isolated from soil in Yamanashi Prefecture. The strain was cultured at 27°C for 4 days on a rotatory shaker in a medium containing 2% oatmeal and 0.1% yeast extract, with 2% vegetative inoculum grown in the same medium for 4 days. The antibiotic was purified via adsorption of the filtrate on Diaion HP20 and elution with MeOH, MeOH extraction of the mycelial cake, combination and concentration of the MeOH solutions, EtOAc extraction after pH adjustment to 7.0, silica gel column chromatography (washed with CHCl3, eluted with CHCl3-MeOH 20:1), preparative TLC (silica gel, EtOAc-MeOH 20:1), and HPLC (C18-Silica, CH3CN-H2O 45:55), yielding ~25 mg of IM8443T (monitored by growth-inhibitory activity against L5178Y murine lymphoma and Bacillus subtilis). IM8443T was a colorless crystalline powder soluble in CHCl3, EtOAc, EtOH, and MeOH but hardly soluble in H2O and carbon tetrachloride. Its physicochemical properties (FD-MS: m/z 684 (M+H)+; UV, IR, 1H NMR) suggested similarity to oxazolomycin, and it was identified as curromycin B by 1H NMR and molecular weight comparison. In vitro, IM8443T inhibited murine tumor cells (L5178Y, L1210, P388) with IC50 0.01~0.1 μg/ml and Gram-positive bacteria (e.g., Staphylococcus aureus IAM1011 MIC 50 μg/ml, Micrococcus luteus IAM1056 20 μg/ml, Bacillus subtilis PC1219 5 μg/ml) but showed no activity against Gram-negative organisms, Mycobacterium, or fungi at 100 μg/ml.