The post-synaptic neurotoxins can be defined as either α-neurotoxins or κ-neurotoxins according to functional differences; however, the sequence and structural homology among the α- and κ-neurotoxins and among their respective nicotinic acetylcholine receptor (nAChR) suggests that the toxins might demonstrate cross-reactivity. This chapter describes the observation that recombinant κ-Bungarotoxin (κ-bgt) has an affinity for the muscle receptor— that is, 1–2 orders of magnitude lower than reported previously for venom derived κ-bgt. The functional comparison of venom-derived and recombinant κ-bgt is predicated on the assumption that the recombinant toxin is properly folded and accurately reflects the native affinity of κ-bgt for the muscle receptor. Both recombinant and venom-derived κ-bgt have shown an identical ability to block the nAChR in the chick ciliary ganglion, indicating that the recombinant toxin had attained its functional conformation. Whether the observed muscle receptor block is due in part to contamination by α-bgt can be answered definitively only by further purifying venom-derived κ-bgt, and showing that the active component in the sample is α-bgt by direct analysis. The chapter describes that potential cross contamination of venom-derived toxins suggested by these observations should be considered in the design and interpretation of any future studies, involving such toxins.