Daboiatoxin (DbTx), a phospholipase A 2 neurotoxin purified from Daboia russelii siamensis venom and radioiodinated to high specific activity (500-700 Ci/mmole), bound specifically with high affinity to synaptosomes from Wistar rat, snake (Python reticulatus), and little civet cat (Viverricula indica). While rat brain synaptosomes contained two different binding sites of high and low affinities (K d l = 5-8 nM; B m a x 1 = 6 pmoles/mg andK d 2 = 80 nM; B m a x 2 = 20 pmoles/mg), synaptosomes from python (K d = 6.2 nM;B m a x = 26 pmoles/mg) and civet cat (K d = 5.8 nM; B m a x = 22 pmoles/mg) exhibited only a single class of high-affinity binding sites. Binding of 1 2 5 I-DbTx to rat synaptosomes was displaceable by 1 μM and above concentrations of crotoxin, mojave toxin, ammodytoxin A and Crotalus atrox PLA 2 , but 1 2 5 I-DbTx bound to python synaptosomes was indisplaceable by those neurotoxins, indicating relatively tighter binding of 1 2 5 I-DbTx by the latter. Competition binding experiments with a range of neurotransmitter antagonists, and potent K + and Ca 2 + channels blockers suggest that the receptor target for DbTx is neither adrenergic, nicotinic and muscarinic cholinergic, GABA, opiate, dopamine, and serotonin receptors, nor any of the subtypes of K + and Ca 2 + channels. DbTx strongly inhibited the uptake of 3 H-choline, whereas 3 H-GABA uptake was not affected. Compared to sera of normal human (control) or civet cat, python serum was equally effective as polyclonal rabbit anti-DbTx antisera in inhibiting 1 2 5 I-DbTx binding (ic 5 0 of both sera = 1:6000 dilutions) to rat synaptosomes, or to python and civet cat synaptosomes. This finding is indicative of the presence of 1 2 5 I-DbTx binding inhibitor in the python serum. Further work in defining this factor is in progress.