We studied the in vitro effects of granulocyte-macrophage colony-stimulating factor (GM-CSF) in 13 patients with acute myeloid leukemia (AML) and one patient with refractory anemia with excess of blasts in transformation using the AML blast (AML colony-forming units, AML-CFU) and mixed (granulocyte erythrocyte macrophage megakaryocyte colony-forming units, CFU-GEMM) colony culture assays. In parallel, these patients received GM-CSF s.c. at 125 micrograms/m2/day, or in escalated doses starting with 10 micrograms/m2/day for a week or until circulating blast counts reached 50 x 10(9)/liter, in an effort to sensitize leukemic blasts to cell-cycle-specific agents. Results of in vivo GM-CSF treatment were correlated with those of in vitro assays. In 9 of 12 patients (75%), GM-CSF treatment increased peripheral blood blast counts (in vivo effect). GM-CSF also stimulated in vitro AML blast colony proliferation in these nine patients and increased the S+G2M phases of the cell cycle in five out of five of these patients' samples. Two of three patients in whom an in vivo response could not be demonstrated also failed to have a detectable in vitro response. These observations suggest that the AML blast colony culture assay may be useful in predicting the response of AML to cytokine therapy. Finally, GM-CSF stimulated granulocyte-macrophage (granulocyte-macrophage colony-forming units, CFU-GM) and erythroid (erythroid burst-forming units, BFU-E) colony proliferation in 14 and 11 patients, respectively, including the 3 individuals who demonstrated no clinical effect on blast counts. It is, therefore, possible that GM-CSF may be used to stimulate proliferation of progenitors that differentiate into mature granulocyte, monocyte-macrophage, and erythroid cells.