The cell cycle age response of 9L rat brain tumor cells to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) was determined using centrifugal elutriation to obtain populations of cells enriched in G1, S, and G2-M phases. While cells in all phases of the cell cycle were killed by 20 or 40 .mu.M cells in G1 and G2-M phases were more sensitive than cells in S phase. The differential sensitivity was more pronounced at the higher dose, which will markedly alter the distribution of cells through the cell cycle. In a clinical setting, this factor could affect the efficacy of either fractionated or multimodality protocols. Treatment with .alpha.-difluoromethylornithine, a polyamine biosynthesis inhibitor, potentiated the cytotoxic effects of 20 .mu.MBCNU against G1- and G2-M- but not against S-phase cells; however, at a higher dose of BCNU (40 .mu.M), the cytotoxicity was potentiated for cells in all phases of the cell cycle. In .alpha.-difluoromethylornithine-treated cells, the phenomenon could be reversed by adding 1 mM putrescine 24 h before treatment with BCNU. Therefore, the potentiation of BCNU cytotoxicity appears to be related to polyamine depletion.