High affinity acceptors for alpha-dendrotoxin, a selective probe for certain fast activating voltage-dependent K+ channels, were purified approximately 4,000-fold from synaptic plasma membranes of bovine cerebral cortex. Although the preparation possessed a low content of high affinity sites for beta-bungarotoxin, antagonism of alpha-dendrotoxin binding by the latter required high concentrations; this indicates that more than one acceptor subtype has been purified. After deglycosylation of the acceptor, the sizes of its subunits were determined electrophoretically to be Mr 65,000 and Mr 39,000. Solid phase microsequencing of these isolated subunits showed that the smaller one had a blocked N terminus, but the Mr 65,000 protein gave a sequence of 27 residues. This is virtually identical to the N-terminal sequence deduced from cDNA of RCK 5, a K+ channel protein from rat brain known to be susceptible to alpha-dendrotoxin. This first report on the partial sequence of any K+ channel protein confirms that the extensive information acquired to date on the alpha-dendrotoxin acceptors is pertinent to functional neuronal K+ channels.