The knowledge about drug resistance in childhood leukemias and acute lymphoblastic leukemia (ALL) in general is limited. This is because of the lack of a suitable in vitro drug sensitivity assay, which is in part due to low in vitro ALL cell survival. We recently adapted the highly efficient 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay to test cells from ALL patients and showed that its results were comparable with those of the DiSC assay, up to now the most valid but laborious assay. In this study, in vitro drug sensitivity was assessed in cells from 82 children with leukemia, 79 of whom had ALL, with the MTT assay. Dose response curves were obtained for 6-mercaptopurine, 6-thioguanine (6-TG), prednisolone (Pred), daunorubicin (DNR), vincristine (VCR), cytosine arabinoside (Ara-C), L- asparaginase (L-Asp), mafosfamide, and mustine. A cytotoxic effect of methotrexate could be detected in only a few cases. Large interindividual differences in drug sensitivity were detected. Compared with leukemia cells from newly diagnosed patients, leukemia cells from relapsed patients were significantly more in vitro resistant to 6-TG, Pred, Ara-C, mafosfamide and mustine but not to DNR, VCR, and L-Asp. Improvements of culture medium and methods to increase MTT reduction were studied. From 10 components tested, addition of insulin and bovine serum albumin to serum-containing medium improved ALL cell survival. Addition of succinate did not increase the amount of MTT reduction. We conclude that the in vitro MTT assay highly facilitates large-scale studies on drug resistance of ALL patients that can lead to rational improvements in existing treatment protocols.