We have previously shown that a factor termed NAP-IF has the capacity to induce neutrophil alkaline phosphatase (NAP) in postmitotic granulocytes (PMGs). Recently, this factor found in cystic fluid of a human squamous cell carcinoma was shown to be identical to granulocyte colony-stimulating factor (G-CSF). In this study we examined whether NAP activity inducible with G-CSF could be modulated by other factors that are present in vivo or those that are known to induce differentiation of hemopoietic cells. Purified natural and recombinant G-CSF (nG-CSF and rG-CSF) induced NAP in PMGs from both normal individuals and patients with chronic myelogenous leukemia. Interferons (IFNs) suppressed expression of NAP by G-CSF. IFN-gamma was a potent inhibitor of G-CSF stimulation: IFN-gamma at 100 U/ml inhibited by greater than 90% the induction of NAP by G-CSF. In contrast, retinol (10(-6) M, a nearly physiological concentration) or all-trans-retinoic acid (10(-6) M) significantly enhanced NAP activity in vitro. Furthermore, the simultaneous addition of 10(-6) M retinol partially reversed the inhibitory action of IFN-gamma on the NAP induction by G-CSF. Our results suggest that NAP activity, which is often abnormal in a variety of diseases, may reflect G-CSF levels in vivo perhaps in concert with a number of other factors including IFNs and retinoids.