alpha-Difluoromethylornithine, a known inhibitor of polyamine biosynthesis, significantly enhanced the cytotoxic effect of 1,3-bis(2-chloroethyl)-1-nitrosourea, a cell cycle-nonspecific agent, in 9L rat brain gliosarcoma cells in vitro. Administered as a single agent, alpha-difluoromethylornithine was not cytotoxic to 9L cells and, compared to untreated control cells, caused no perturbation of cell cycle kinetics. alpha-Difluoromethylornithine-induced depletion of intracellular polyamine levels appears to have caused the observed sensitization of 9L cells to 1,3-bis(2-chloroethyl)-1-nitrosourea. Restoration of intracellular polyamine levels by the addition of exogenous putrescine to treated 9L cells reversed this phenomenon.