In order to further evaluate the antitumor activity of 1-(2-chloroethyl)-3-isobutyl-3-(beta-maltosyl)-1-nitrosourea (TA-077), the therapeutic ratios (a ratio of the optimal dose versus the dose showing 25% increase in life span) of TA-077 were examined using two L1210 leukemia systems and the results were compared with those of twenty-six antitumor agents for reference. In the i.p.-i.p. system, in which the tumor cells were inoculated intraperitoneally (i.p.) and the drugs were also administered i.p., TA-077 administered daily for five consecutive days showed a therapeutic ratio of 40.6, which was the third highest of all drugs tested. However, in the i.v.-i.v. system, in which the tumor cells were inoculated intravenously (i.v.) and the drugs were also administered i.v., TA-077 given on the same schedule showed a ratio of 13.7, which was the highest of all the drugs examined. The results of the present study also indicated that L1210 leukemia cells were highly sensitive to nitrosourea derivatives. Therefore, this should be taken into consideration for screening of candidates for new antitumor agents.