AML cells were cultured free of serum with G-CSF in combination with all-trans-retinoic acid (RA), prostaglandin E2 or 8-bromocyclic AMP to see whether the maturation blockade of these cells could be overcome. The combination G-CSF + RA was most effective in inducing morphologic maturation, i.e. in 7/10 cases. Morphological alterations in response to G-CSF + RA indicated progression of the cells along the granulocytic pathway towards metamyelocytes and granulocytes. However, morphologically mature AML cells remained negative for myeloperoxidase and Sudan black stainings, indicators of granulocytic maturation. Chloracetate esterase positivity and CD15 membrane antigens became expressed on cultured AML cells, i.e. on unstimulated and G-CSF/RA exposed blasts. Ingestion of latex beads and reduction of nitroblue tetrazolium salt occurred in cultured AML cells regardless of the presence of inducers. In almost all cases clonogenic cells persisted after exposure to G-CSF + RA suggesting that subpopulations of immature cells escaped the action of these inducers. Thus although G-CSF + RA were capable of inducing maturation of AML cells along the granulocytic lineage, maturation was incomplete and the effect was evident in a subfraction of the cells only.