Predictive capacity and clinical usefulness of the short term predictive assay (STPA) in the inductive treatment of patients with acute nonlymphoblastic leukemia (ANLL) was studied. Inductive treatment consisted of daunorubicin, arabinoside C and 6-thioguanine (TAD regimen). Leukemic cells of 20 previously untreated patients with ANLL were incubated in vitro with two doses of daunorubicin (1 microgram/ml and 10 micrograms/ml), arabinoside C (10 micrograms/ml and 100 micrograms/ml) and 6-thioguanine (10 micrograms/ml and 100 micrograms/ml). The 3H-thymidine as well as 3H-uridine uptake was measured in the treated and untreated cells. The highest predictive presence of the in vivo drug-sensitive disease was adequately reflected by the level of 3H-uridine incorporation suppression 30% of control value in the case of daunorubicin (concentration: 10 micrograms/ml) and 80% in the case of 6-thioguanine (concentration: 100 micrograms/ml). In the case of arabinoside C (concentration: 10 micrograms/ml) the limit of 3H-thymidine uptake depression was 20% of control value. It was rather difficult to define the indicative degree of precursors incorporation inhibition for prediction of the drug-resistant disease, because of low number of patients primary resistant to TAD regimen. No correlation was found between the degree of the pre-treatment DNA synthesis rates and the precursors uptake inhibition by the tested drugs.