In the present study we evaluated the effect of etoposide (VP-16-213) compared to mafosfamide-cyclohexylamine (Asta-Z 7654) on normal granulocyte-macrophage colony-forming unit (GM-CFU) growth, T-cell response to mitogens, and a clonogenic promyelocytic cell line (HL-60). The incubation time (30 min vs 60 min) appeared to be a fundamental parameter. The GM-CFU recovery was 14.4% +/- 7.3% and 1.4% +/- 2.3%, respectively, at 50 micrograms/ml Asta-Z 7654, and 17.6% +/- 8.6% and 3.00% +/- 2.4%, respectively, at 50 micrograms/ml VP-16. ASTA-Z at 50 micrograms/ml was effective in inhibiting the T-cell response to phytohemagglutinin (98.7% +/- 1.2%), whereas VP-16 was not (2.3% +/- 1.7%). With the combined chemical agents ranging from 10 to 20 micrograms/ml for each drug, we obtained a better GM-CFU recovery (five to ten times) using a middle term liquid culture (21-day incubation) than with the standard colony assay (plated immediately after treatment). When using HL-60 cells as the target, the antileukemic activity of VP-16 was lower than of Asta-Z 7654. Both compounds, at 20 micrograms/ml, resulted in 3.3- and 2.3-log cell killing, respectively. On the other hand, lower doses of Asta-Z 7654 combined with VP-16 (ranging from 10 to 15 micrograms/ml each) induced greater than 4-log cell killing after 60 min incubation time. These data suggest that VP-16 could be combined with Asta-Z provided that the dose is reduced for both drugs (less than 20 micrograms/ml).