A voltage-activated inward-rectifying K+ conductance (lKi) appears in human promyelocytic leukemia (HL-60) cells during phorbol ester-induced differentiation into macrophages. This conductance was detected in the cells 24 hours after exposure to phorbol-12-myristate-13-acetate (PMA), as the cells began to express the macrophage phenotype, and continued to increase for 4 days after PMA exposure. The magnitude of inward current was a function of external K+; current was blocked by extracellular or intracellular Cs+ and by extracellular Ba++. Hyperpolarization produced activation at membrane potentials more negative than -80 mV, and a slower, partial inactivation also occurred at potentials more negative than -100 mV. This conductance was not detected in proliferating cells nor in granulocytes derived from HL-60 cells which were induced to differentiate with retinoic acid (RA). Exposure of differentiated macrophages to recombinant human CSF-1 produced inhibition of the lKi beginning within 1 minute after exposure. CSF-1 inhibition of lKi channels in cell-attached patches indicated that channel modulation was via intracellular mediators. The rapid inhibition of the inward rectifier by the macrophage-specific CSF-1 appears to be one of the earliest cellular responses to this factor.