The effects of ellipticine [5,11-dimethyl-6H-pyrido(4,3-b)carbazole; NSC 71795] on cell viability, growth, and colony formation were investigated in suspension (Friend leukemia and L1210) and adherent [Chinese hamster ovary (CHO)]tumor cell systems as well as in mitogen-stimulated human peripheral blood lymphocyte cultures. Cell cycle progression and the terminal point of action of the drug were monitored by flow cytometry. Ellipticine was cytostatic for all cell lines tested, blocking cells in G2 phase following 24 hr constant exposure at concentrations in the range of 1.0 microgram/ml. A 10 times higher drug concentration was required to block cells in G2 if the cells were exposed for only 30 min to the drug followed by 23.5 hr culture in drug-free medium. Formation of CHO cell colonies was inhibited by 50% following exposure to ellipticine for 2 hr at 6.0 microgram/ml or for 24 hr at 0.3 microgram/ml. Fifty % cell kill in asynchronously growing Friend leukemia and L1210 cells was obtained following exposure to ellipticine for 24 hr at 2.0 microgram/ml and 1.15 microgram/ml, respectively, whereas human peripheral blood lymphocytes required 66 hr exposure to 1.0 microgram/ml to kill 50% of the cells. Phytohemagglutinin-stimulated lymphocytes were remarkably resistant to the cytotoxic effect of ellipticine but did display a dose-dependent inhibition of stimulation and accumulation in G2 whether the drug was added prior to our during active cell proliferation. Ellipticine, at cytostatic concentrations, had a marked effect on cellular RNA content. Friend leukemia cells, blocked in G2 by the drug, doubled their RNA content compared to control cells. L1210 and CHO cells, but not lymphocytes, also increased in RNA content following ellipticine treatment. Drug concentrations which blocked cells in G2 also led in the case of Friend leukemia and L1210 but not CHO cells to an increase in the proportion of cells with greater than 4C amounts of DNA.