The effects of the L isomer (+)-1,2-bis(3,5-dioxopiperazine-1-yl)propane (ICRF 159; NSC 169780) on cell viability, growth, and progression through the cell cycle were investigated in suspension cultures of murine leukemia (Friend leukemia and L1210) cells and normal human lymphocytes stimulated with phytohemagglutinin and in adherent cultures derived from human neuroblastoma and Chinese hamster ovary (CHO) cells. CHO cell colony formation was inhibited by 50% following either an 8.5-hr exposure of exponentially growing cells to 10 micrograms ICRF 159 per ml or a 24-hr exposure to 3 micrograms ICRF 159 per ml. This effect was cell cycle phase specific; early G1- and G2-phase cells were more sensitive than were late-G1- or early and mid-S-phase CHO cells. Stationary-phase CHO cells were unaffected by the drug at concentrations up to 500 micrograms/ml. Incubation of L1210 cells with 3 micrograms ICRF 159 per ml for 24 hr or with 10 micrograms ICRF 159 per ml for 6 hr inhibited cell growth by 50%. In contrast, 24-hr incubation of human lymphocytes with up to 50 micrograms ICRF 159 per ml had no effect on their viability or on their ability to be stimulated by phytohemagglutinin. Constant exposure of Friend leukemia, L1210, human neuroblastoma, and phytohemagglutinin-stimulated human lymphocytes to 10.0 to 50 micrograms ICRF 159 per ml resulted in inhibition of cell division which led to cell growth at higher ploidy levels. Thus, proliferating human cells of normal or tumor origin and murine leukemic cell lines all had a similar sensitivity to the drug. Detailed analysis of cell cycle progression in L1210 cells in the presence of the drug determined that cell progression through G1 phase (G1A to G1B transition) was slowed by approximately 50%. The rate of traverse of cells through S phase was also slowed. However, the most pronounced effect was the accumulation of cells in G2 phase occurring almost immediately after addition of the drug. The data suggest that the L isomer has a range of cytotoxicity and identical cytokinetic effects similar to that of the clinically tested racemate (+/-)-ICRF 159 (NSC 129943) and, therefore, that the more soluble L isomer may have increased clinical applicability.