Regulatory molecules that affect the growth culture of blast cells from acute myeloblastic leukemia (AML) may also alter drug sensitivity, a phenomenon that may be called regulated drug sensitivity. Previous studies have shown: (i) blast cells exposed to retinoic acid before cytosine arabinoside (Ara-C) usually show increased sensitivity, but after some retinoic acid exposure times, sensitivity may be decreased; (ii) factor-sensitive or responsive blasts cultured with granulocyte colony-stimulating factor (G-CSF) are regularly more Ara-C-sensitive than when cultured with granulocyte-macrophage CSF (GM-CSF). This paper is concerned with the effects of schedule on drug sensitivity as regulated by either retinoic acid or the myelopoietic growth factors, G-CSF and GM-CSF. We measured the effects of retinoic acid on the sensitivity of blasts cells from the two continuous AML lines to Ara-C or arabinofuranosyl 5-azacytosine (Ara-AC). Cells from seven patients with AML were tested for Ara-C sensitivity in conjunction with retinoic acid. The cells were treated with retinoic acid before or after administration of the drug. Both increases and decreases in Ara-C sensitivity were seen for both schedules. Consistent increases in Ara-C sensitivity were obtained when retinoic acid was included in the methylcellulose cultures used to determine clonogenic cell recovery at each drug dose. In studies of growth factors, a single factor-dependent cell line (OCI/AML-5) was used to compare the effects of G-CSF and GM-CSF on Ara-C sensitivity. An experimental design was used that permitted factors to present in culture for 24 h before Ara-C, during the next 24 h period with the drug, for a subsequent day in suspension without drug, and during the 5-7 days required for colony formation in methylcellulose cultures. G-CSF and GM-CSF were most effective in increasing or decreasing Ara-C, respectively, when the factor under test was included in the methylcellulose cultures. Thus, like retinoic acid, growth factors influenced drug sensitivity when they were present after the drug had been removed. These data, therefore, are compatible with the hypothesis that repair mechanism may contribute to regulated drug sensitivity.