IsK is a novel membrane protein that induces a slow voltage-gated K+ current when expressed in Xenopus oocytes. Attempts were made to express this channel in several eukaryotic cells including the skeletal muscle cell line C2C12, the fibroblastic cell line CHO and the T lymphocyte cell line Jurkat. In spite of the clear demonstration of successful transfection, no IsK current could be recorded from these cells. Overexpression was also obtained using the vaccinia/T7 and baculovirus systems, again without any success in demonstrating the presence of the K+ current in the plasma membrane of infected cells. Bilayer reconstitution experiments using membranes highly enriched in IsK protein ruled out the possibility that the IsK protein could induce a K+ channel located in intracellular organelles. Collectively, these data suggest the possibility that IsK protein is part of a K+ channel complex but is only active in association with another protein endogenous to the Xenopus oocyte. Other hypotheses are also discussed.