Voltage-dependent K+ channels are responsible for repolarization of the cell membrane during the late phase of the action potential. Here we report the purification of proteins from squid axon membranes which bind the K+-channel blocker noxiustoxin (NTX), and their subsequent functional reconstitution in planar bilayers. The NTX-affinity purified proteins had Mr values of 60,000 +/- 6,000, 160,000 +/- 15,000 and 220,000 +/- 20,000. Their incorporation into bilayers resulted in single-channel currents with three conductances, the most frequent one of 11 pS in 300/100 mM KCl (cis/trans). The voltage dependence, reversal potential and bursting behavior suggest that these are the K+ channels involved in the squid axon action potential.