In vitro chemosensitivity of blast cells from 19 patients, affected by acute non-lymphocytic leukemia, to cytosine arabinoside and daunorubicin was investigated. A semiautomated P-iodonitrotetrazolium violet (INT) colorimetric assay, based on the ability of viable cells to cleave piodonitrotetrazolium violet into a red formazan derivative and a short-term antimetabolic assay based on the uptake inhibition of [3H]-thymidine were compared in terms of their ability to predict clinical outcome. Both methods were able to discriminate between sensitive and resistant patients by in vitro cytosine arabinoside testing. On the contrary the assays carried out with daunorubicin failed to predict the clinical outcome. A good correlation was demonstrated between the results of the two different methodological approaches, validating the recently described INT sensitivity test.