A novel 15-methoxyansamitocin P-3 121, in company with three known maytansinoids 1, 3, and 4, was isolated for the first time from two Japanese mosses, Isothecium subdiversiforme and Thamnobryum sandei. All are potent cytotoxic compounds. During our study on the isolation of the components with biological activity in mosses (l), we searched for antitumor components utilizing the screening of mosses by in vitro cytotoxicity tests against mouse P-388 lymphocytic leukemia cells. Among a large number of mosses examined, crude MeOH extracts of two mosses, Isothecium subdiversiforme Broth. (Lembophyllaceae) and Thamnobryum sandei (Besch. ) Iwatsuki (Neckeraceae), had activity (50% cell growth inhibitory concentration, IC,, 2-5 kg/ml). These mosses are seen commonly in the southern half of Japan. Every sample of I. subdiversiforme collected from six locations in Japan revealed cytotoxic activity. Moreover, a moss extract that was concentrated to 100 times more in vitro activity showed a moderate degree of in vivo activity against mouse P-388 cells (prolonged survival interval T/C 149%, day 1-5, ip, dose 40 mg/kg).The results of activity tests prompted us to carry out isolation of the active cytotoxic principles from the mosses. As a result, we confirmed the components as four maytansinoids, including a novel 15-methoxy derivative of ansamitocin P-3. Maytansinoids are ansamacrolides that were isolated first by Kupchan et ai. (2) in 1972 from an African shrub Maytenus serrata in very low yield. They are potent cytotoxic substances with excellent activity in mouse P-388 leukemia in vivo tests (2-4). The subsequent plant screening program operated by NCI revealed the existence of a number of biologically active maytansinoid derivatives. Besides the plant origin, Asai et al. (5) reported in 1979 the production of ansamitocins with maytansinoid skeleton by a microorganism of Nocardia species as the potent antitumor components.From mosses in which maytansinoids had not previously been reported, we have isolated and identified four maytansinoids that exhibit a potent cytotoxicity.