In vitro clonogenic assays may be useful for determining the sensitivity of leukemic cells to chemotherapeutic agents. We evaluated the antileukemic effect of Bisantrene (an anthracene derivative now undergoing phase II clinical trials in relapsed/resistant acute non lymphoid leukemias-ANLL) using the ANLL cell clonogenic assay. Fifteen cases were studied (9 newly diagnosed, 5 relapsed and 1 refractory ANLL). Normal CFU-GM sensitivity was tested in a subset of 10 normal controls. A wide range of concentrations (from 0.01 to 10 micrograms/ml) at 3 durations of exposure (30 min, 120 min, continuous) was employed. Bisantrene proved effective in 12 out of 15 ANLL cases, inhibiting blast colony growth (50% at 1 micrograms/ml; nearly 100% at 10 micrograms/ml) in a dose-dependent, time-independent way. Three cases were unresponsive both in vitro and in vivo. Normal CFU-GM were inhibited at lower doses (50% at 0.5 micrograms/ml; 100% at 5 micrograms/ml). We conclude that: 1) Bisantrene is active in vitro on leukemic clonogenic cells at doses corresponding to plasmatic levels achievable in patients, with a parallel activity in vivo in 3 relapsed cases. It should be tested in vitro before therapeutic use in order to avoid, if possible, improper use in resistant patients. 2) Normal CFU-GM are more sensitive than clonogenic leukemic cells. This must be taken into account, in view of possible prolonged neutropenias after therapy. 3) The time-independent effect of the drug should be evaluated in the design of new therapeutic schedules.