Voltage-dependent Shaker-related (Kv1) K+ channels are composed of transmembrane alpha subunits and peripheral Kv beta proteins that exist as octomers with (alpha)4(beta)4 stoichiometry. Although several alpha (designated Kv1.X) and three Kv beta subunits are known to be expressed in brain, their oligomeric combinations in neurons have yet to be deciphered. Herein, the subunits comprising a number of neuronal K+ channels from bovine brain cortex were deduced by immunoprecipitation and Western blotting, using antibodies specific for Kv1.X and Kv beta subtypes. Only a subset of the theoretically possible oligomers was detected, showing that the synthesis and/or assembly of these multisubunit K+ proteins is controlled to yield a limited variety of K+ channels. Except for a small population of Kv1.4 containing K+ channels, all the recognizable species contained Kv1.2 and beta2 subunits. Furthermore, several subpopulations were identified including a fully defined complex of Kv1.2/1.3/1.4/1.6 and Kv beta2, plus oligomers containing three or two assigned alpha subunits. Kv1.2 was also shown to occur in the absence of these other subunits as a putative homo-oligomer. Thus, for the first time, the complete subunit combination of an authentic K+ channel has been elucidated; also, the strategy employed to establish this can now be applied to closely related members of other K+ channel families.