Properties of inwardly rectifying K+ channels in small-cell lung cancer (SCLC) cells have not been clarified in detail. Here, we found inwardly rectifying K+ channels in a human SCLC cell line (RERF-LC-MA), which expresses no multidrug resistance-associated protein 1 (MRP1) and multidrug resistance P-glycoprotein (MDR1). Extracellular Ba2+ and Cs+ inhibited inwardly rectifying K+ currents of RERF-LC-MA cells in a concentration-dependent manner, but tetraethylammonium ion and glibenclamide were ineffective. Okadaic acid, an inhibitor of phosphatases 1 and 2A, and phorbol-12,13-dibutyrate, an activator of protein kinase C, significantly decreased the inwardly rectifying K+ current. Lowering the intracellular pH but not the extracellular pH decreased the K+ current. Reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting analysis showed that RERF-LC-MA cells express Kir2.1 mRNA and protein. The inwardly rectifying K+ current is suggested to be generated by Kir2.1 protein in the human small-cell lung cancer cell, and that the K+ channel is negatively regulated by protein kinase C and the intracellular acidic pH.