In the course of screening for new antitumor substances, a novel cytotoxic agent inducing apoptotic cell death was isolated from the culture broth of marine bacterial strain SCRC A-20. Strain SCRC A-20 was separated from a mollusk and was chemotaxonomically identified as a Streptomyces sp. The cytotoxic substance was purified by organic solvent extraction followed by silica gel column chromatography and preparative TLC. HRFAB-MS determined its molecular formula to be $C_{30}H_{40}N_2O_5$ (MW 508). The 1D and 2D NMR spectral data demonstrated that the substance has a novel lactam structure of a 20-membered macrocycle coupled with a unique acyl tetramine and bicyclo［3.3.0］ octane, which includes three methyl groups, six olefinic protons, five carbonyl groups, a conjugated diene and a dienone. The substance, named aburatubolactam C, appeared to be cytotoxic for various continuously proliferating tumor cells of human and murine origins. The $IC_{50}$ values determined by MIT assay were in the range of 0.3 to $5.8mug/ml$. When Jurkat T cells were treated with $3mug/ml$. of aburatubolactam C, the apoptotic DNA fragmentation was detectable within 3 h, indicating that the cytotoxic effect of aburatubolactam C on tumor cells is attributable to the induced apoptosis.