The mast cell degranulating peptide (MCD) and dendrotoxin I (DTXI) are two toxins, one extracted from bee venom, the other one from snake venom, that are thought to act on voltage-sensitive K+ channels. Binding sites for the two toxins have been solubilized. The solubilized sites were stable and retained their high affinity for 125I-DTXI and 125I-MCD (Kd approximately equal to 100 pM). Interactions were found between MCD and DTXI binding sites in the solubilized state, establishing that the two different toxins act on the same protein complex. This conclusion was strengthened by the observations (i) that conditions of solubilization that eliminated 125I-MCD binding activity also eliminated 125I-DTX binding activity while both types of activities were preserved in the presence of K+ or Rb+ and (ii) that binding components for the two types of toxins had similar sedimentation coefficients and copurified in partial purifications. A component of the receptor protein for 125I-MCD has been identified; it has a Mr of 77,000 +/- 2000. This polypeptide was similar to or identical in molecular weight with that which serves as a receptor for DTXI (Mr 76,000 +/- 2000).