FR901463, FR901464 and FR901465, novel antitumor substances, were isolated from the fermentation broth of Pseudomonas sp. No. 2663. Their antitumor activities were examined in three mouse tumor systems and one human tumor system. The three FR compounds prolonged the life of mice bearing murine ascitic tumor P388 leukemia (T/C values were 160%, 145% and 127% for FR901463, FR901464 and FR901465, respectively), and inhibited the growth of a human solid tumor, A549 lung adenocarcinoma, with different effective dose ranges. FR901464 exhibited most prominent effects on these tumor systems among the three FR compounds. FR901464 also inhibited the growth of murine solid tumors, Colon 38 carcinoma and Meth A fibrosarcoma. To address the involvement of transcriptional activation ability of the three FR compounds in the antitumor effect, we selected FR901464 as a candidate compound and investigated cell cycle transition, chromatin status and endogenous gene expression in FR901464-treated tumor cells having elevated transcriptional activity. FR901464 induced characteristic G₁ and G₂/M phase arrest in the cell cycle and internucleosomal degradation of genomic DNA with the same kinetics as activation of SV40 promoter-dependent cellular transcription in M-8 tumor cells. In contrast to the potent activation of the viral promoter, FR901464 suppressed the transcription of some inducible endogenous genes but not house keeping genes in M-8 cells. These results suggest that FR901464 may induce a dynamic change of chromatin structure, giving rise to strong antitumor activity, and therefore may represent a new type of drug for cancer chemotherapy.