Fungal metabolites with an epz-oligothiadiketopiperazine structure, TAN-1496 A, C and E, were isolated from the culture broth of Microsphaeropsis sp. FL-16144. Their molecular formulas were determined to be C22H28N2O9S2,C22H28N2O9S3and C22H28N2O9S4,respectively. Structures were determined by comparing the NMRdata with those of known diketopiperazine antibiotics, sirodesmins. These metabolites inhibited the relaxation of supercoiled pBR322DNAby calf thymus topoisomerase I but did not affect the decatenation ofkinetoplast DNAby calf thymus topoisomerase II at concentration up to 500 /j.m. They strongly suppressed the growth of various murine and human tumor cells and induced apoptosis. Moreover, various derivatives were synthesized to investigate the relationship of their functional groups and biological activities.