Crotoxin, the major toxic component of the South American rattlesnake, Crotalus durissus terrificus, is a neurotoxic phospholipase A2 which exerts its pathophysiological action by blocking the neuromuscular transmission. Crotoxin acts primarily by altering the acetylcholine release from the nerves terminals through a mechanism which has not yet been elucidated. It also acts on postsynaptic membranes by stabilizing the acetylcholine receptor in an inactive conformation very similar to the desensitized state. Crotoxin is made of two dissimilar subunits: a basic and weakly toxic phospholipase A2 component-B, and an acidic and non toxic component-A which does not possess any enzymatic activity. Binding experiments showed that crotoxin subunits dissociate when crotoxin interacts with biological membranes: Component-B binds, whereas component-A appears free in solution. The phospholipase A2 subunit binds in a non saturable, non specific manner, on any kind of biological membranes, whereas in the presence of component-A it interacts only with a limited number of high affinity binding sites present on synaptic membranes but not on erythrocyte membranes. Although the target site (acceptor) of crotoxin has not yet been formally identified, binding experiments carried out with small unilamellar phospholipid vesicles of different compositions indicate that some negatively charged phospholipids like mono and diphosphoinositide phosphates might be an important component of crotoxin acceptor site. Crotoxin is in fact a mixture of several isoforms which have very similar but not identical polypeptide sequences. An individual Crotalus durissus terrificus snake is able to synthesize several crotoxin isoforms which may result of the expression of several isogenes and/or of post-translational events. When compared in quantitative manner, the crotoxin isoforms slightly but significantly differ in their enzymatic and pharmacological properties. Finally, immunochemical investigations carried out with polyclonal antibodies prepared against both crotoxin subunits, showed that non precipitating anti-component-B- antibodies (Fab) inhibit the phospholipase A2 activity of crotoxin and neutralize its lethal potency, suggesting that the catalytic and toxic sites of crotoxin are closely related.