The in vitro chemosensitivity to daunorubicin and cytosine arabinoside of blast cells derived from 35 patients affected by acute non-lymphocytic leukemia was assessed by a semiautomated tetrazolium-based colorimetric assay, by the use of piodonitrotetrazolium violet. The results of the in vitro testing were then compared a posteriori to clinical outcome of patients, who followed a schedule of therapy which always included the drugs tested in vitro. Three dosages of drugs were employed to allow the determination of a dose-yesponse curve, which was obtained for all the patients. The data collected in INT assay correlated with the clinical sensitivity of patients, evaluated in terms of achievement of complete remission. For the dosage ofARA-C 500 ng/ml it was possible to establish a significant cutoff between responders and non-responders to therapy, while an acceptable distribution of sensitivity/resistance prediction was found for DNR 500 ng/ml and 5 pg/ml. Present results, together with rapid and easy execution of the test, encourage the use of INT assay in screening leukemic patients' sensitivity to antiblastic drugs before treatment or, in case of resistance to classical chemotherapy, in detecting individual sensitivity to alternative drugs.