The cisplatin-resistant variant of the mouse mammary tumour cell line EMT6/P was derived by continuous in vitro exposure to increasing concentrations of cisplatin over a period of 6 weeks. EMT6/CPR was shown to express 2.5-fold resistance to cisplatin as compared with the sensitive parent line. Development of drug resistance was accompanied by a 1.7-fold increase in cellular glutathione (GSH). Growth for 4 weeks in the absence of cisplatin was reported to lead to loss of resistance phenotype and elevated GSH levels in the resistant cell line EMT6/CPR.