It is known that production in a host of antibodies against a protein is associated with various molecular events. These include the stimulation of specific T-lymphocytes, a step that implies the processing of the protein into peptides by various endosomal/lysosomal enzymes, such as cathepsins. Strikingly, however, we observed in vitro that cathepsins B and D have no degrading effect on toxin alpha from Naja nigricollis, a curaremimetic toxin of 61 amino acids and four disulphides. In sharp contrast, the enzymes exert an efficient cleavage of the toxin polypeptide chain once the toxin disulphides are reduced. We also found that the fully reduced toxin and the native toxin were presented with comparable efficiency to two different T-hybridomas by antigen-presenting cells (APC). Together, the data suggest that presentation of toxin fragments to T-cells requires a reduction step of toxin disulphides and, in agreement with previous findings, that this step may be achieved by APC. We wish to suggest that this phenomenon may commonly occur for any toxic proteins that contain disulphides.