The effects of retinoic acid (RA) on the proliferation of acute myeloblastic leukemia (AML) cells were studied. AML samples were divided into three groups. Namely, RA stimulated blast colony formation by AML samples in group A and inhibited that by the samples in group B, regardless of added growth factors. For the samples in group C, RA inhibited the colonies formed by granulocyte colony-stimulating factor (G-CSF) but stimulated those by granulocyte macrophage CSF (GM-CSF). To investigate the mechanism involved, the effects of RA on growth factor receptors on AML cells were examined by flow cytometry using fluorolabeled ligands. For the samples in groups A and B, RA affected neither G-CSF receptor (GR) nor GM-CSF receptor (GMR). For the samples in group C, exposure to 10(-7) M RA for 1 day clearly increased GMR, but did not affect GR. This finding supports the hypothesis that the increase of GMR is one of the causes of the stimulative effects of RA on cells cultured with GM-CSF in group C.