The uptake of cis-[14C]dichloro(ethylenediamine)platinum(II) (cis-DEP) is reduced in cis-diamminedichloroplatinum(II) (cis-DDP)-resistant L1210 cells [L1210/DDP (SRI)] in comparison to cis-DDP-sensitive L1210 cells (L1210/0). A difference in uptake is observed as early as 6 min after addition of cis-[14C]DEP and increases to approximately 3-fold after 30 min. This reduction in uptake is reflected in a decrease in the binding of platinum to DNA isolated from cis-DDP-treated cells as measured by atomic absorption spectroscopy and in the quantity of intracellular cis-DEP metabolites as measured by high-performance liquid chromatography. Similar differences in uptake are observed between ascitic L1210/0 and L1210/DDP (SRI) cells in vitro and in vivo, showing that the differences in uptake are not due to an artifact of the culturing process. The differences in cis-DEP uptake between the two cell lines were relatively unchanged during 7 mo in culture; however, both cell lines exhibited altered sensitivities to cis-DEP during extended culturing. No difference was observed in the efflux of cis-DEP by the two cell lines. Similarly, no difference in nonprotein and total sulfhydryl contents was observed between L1210/0 and L1210/DDP (SRI) cells. The difference in uptake (3-fold) of cis-DEP between L1210/0 and L1210/DDP (SRI) cells may not account fully for the observed differences in sensitivity of the two cell lines to cis-DDP (18-fold) and cis-DEP (19-fold). A portion of the resistance may be due to differences in the capacity of the two cell lines to survive in the presence of platinum damage.