We recently isolated a family of three closely related mouse K+ channel genes (MK1, MK2, and MK3) with coding regions contained in single uninterrupted exons. Here we have used patch-clamp recordings from Xenopus oocytes injected with mRNA to show that MK3 encodes a channel with biophysical and pharmacological properties indistinguishable from those of voltage-gated type n K+ channels in T cells. In addition, we used the polymerase chain reaction to demonstrate the presence of MK3 mRNA in T cells. These data suggest that MK3 may encode the T-cell voltage-gated type n K+ channel. We also show that MK3 and MK2 are localized on human chromosomes 13 and 12, respectively.