The effects of two second generation platinum drugs, cis-diammine-1, 1-cyclobutane dicarboxylate platinum(II) and cis-dichloro-trans-dihydroxybis(isopropylammine)platinum(IV) , were studied on thymocyte nucleosomes, calf thymus DNA, and intact murine thymocytes. In contrast to cis-dichlorodiamineplatinum(II) (cis-DDP), the binding of cis-diammine-1,1-cyclobutarol dicarboxylate platinum(II) or cis-dichloro-trans-dihydroxybis(isopropylammine)platinum(IV) to nucleosomes or DNA was markedly diminished at commonly used pharmacological doses. Since comparable amounts of the drugs were bound to whole thymocytes, we investigated possible membrane sites of action. The fluorescent probes diphenylhexatriene and trimethylammoniumdi-phenylhexatriene were used to measure membrane fluidity. In the whole cell, marked decreases in anisotropy constants of 1-(4-trimethylammonium)-6-phenyl-1,3,5-hexatriene-p-toluene sulfonate were observed after treatment with cis-diammine-1,1-cyclobutarol dicarboxylate platinum(II) and cis-dichloro-trans-dihydroxybis(isopropylammine)platinum(IV) , but not cis-DDP, at 37 degrees C at therapeutic drug concentrations. This effect was observed only in intact thymocytes, not in isolated plasma membranes or liposomes treated similarly with the drugs. The fluorescent properties of the phospholipid probe, 1-acyl-2-(N-4-nitrobenzo-2-oxa, 1,3-diazole)aminocaproylphosphatidylcholine, were altered only by cis-DDP, whereas those of 1-acyl-2-(6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]caproyl ) phosphatidylethanolamine were altered by all three drugs. The increase in the fluorescence intensity per cell after reaction with the drugs was measured with the flow cytometer. The results suggest that alterations in membrane fluidity are produced by the hydrophobic second generation drugs, whereas changes observed via the probe based on phosphatidylcholine were produced only with cis-DDP. Since cis-DDP has a known avidity for proteins, the observed effects may be related to an altered protein-phospholipid (phosphatidylcholine) interaction. The changes in membrane structure described here may relate directly to cytotoxicity or may effect changes enabling the drug to enter the cell to find its intracellular targets.