In order to evaluate the predictive potential of a short-term antimetabolic assay for acute non-lymphocytic leukemia cells, the activity of daunomycin and cytosine arabinoside in suppressing [3H]-uridine uptake was tested in peripheral blood samples from 31 patients. Independently of the in vitro results, the patients were treated with standard combinations including the two drugs or with cytosine arabinoside alone. The clinical response to chemotherapy, and the in vitro [3H]-uridine uptake inhibition were compared retrospectively. A significant decrease of [3H]-uridine uptake after in vitro exposure to both drugs occurred in 12 patients who achieved complete remission after combination therapy, and was particularly evident for 5 patients who needed only one course of therapy. A similar result was obtained in patients treated with cytosine arabinoside alone. This method may represent a useful tool in clinical practice, by indicating a fraction of leukemic patients particularly sensitive to therapy.