As an experimental model for resistance to cis-diamminedichloroplatinum(II) (cis-DDP), murine leukemia L1210 cells have been exposed to a stepwise increase in cis-DDP concentration to produce a variety of resistant cell lines. Intraspecies hybrids of the sensitive and resistant cells were made to determine whether cis-DDP resistance is a dominant or recessive trait. Hybrid cells displayed a partial degree of resistance as compared to the parental cells. To determine whether this was due to a single codominant trait or contribution from a variety of resistance mechanisms, the cells and hybrids were investigated for alterations in the accumulation of drug, as well as alterations in glutathione levels which might inactivate the drug. The cis-DDP-resistant cells demonstrated both a 50% reduction in accumulation of drug and a 1.7-fold increase in intracellular glutathione. Reducing the glutathione levels in these cells with buthionine sulfoximine did not sensitize them to cis-DDP. The hybrid cells had the same accumulation and the same levels of glutathione as the cis-DDP-sensitive cells. Parallel studies were performed with cells resistant to 1,2-diaminocyclohexaneplatinum(II) analogues. These cells also demonstrated reduced drug accumulation but no increase in glutathione. Therefore, both a decrease in accumulation and increase in glutathione may mediate resistance. Both mechanisms represent recessive traits as demonstrated in the cell hybrids. These mechanisms can only account for a small part of the resistance in these cells. A major, dominant mechanism occurs after the DNA has been platinated, but it remains to be determined whether this involves DNA repair, postreplication repair, or some other as yet unidentified process.