Subunit associations of Pseudocerastes fieldi neurotoxin and rattlesnake presynaptic neurotoxins, as well as interactions of individual acidic and basic subunits from these heterodimeric complexes, have been examined by gel-filtration and light scattering techniques. Substantial differences in elution volumes from gel-filtration columns can result from pH solvent changes, addition of Ca 2 + ions, and anion differences, suggesting changes in subunit interactions. Light scattering measurements carried out in parallel, however, suggest that in most cases elution changes are best explained by altered binding of proteins to the gel filtration matrix and not by changes in subunit association states. Subunit associations in each toxin have ionic and hydrophobic components. Intact mojave toxin is highly resistant to trypsin digestion, but the basic subunit by itself is not. Initial cleavage of the basic subunit occurs at the end of the N-terminal helix (Arg-14), suggesting that this region is protected from trypsin digestion by association with the acidic subunit. Using these results, the primary sequence of each toxin's subunit, our knowledge of the structure of the pro-acidic crotoxin subunit, immunological results, and modeling of crotalid PLA 2 s--based on the structure of Crotalus atrox PLA 2 --we have proposed a model for rattlesnake heterodimeric subunit interactions. Surface regions of the acidic and basic subunits likely to be involved in subunit associations are identified. Since P. fieldi toxin basic subunit does not have the same hydrophobic and basic regions as crotoxin basic subunit, and P. fieldi toxin acidic subunit does not have the same hydrophobic and acidic regions as crotoxin acidic subunit, the two types of association probably have little in common. However, acidic subunit binding to part of the basic subunit interfacial binding surface may provide a similar mechanism for potentiation of toxicity. This exercise provides a working model for heterodimeric toxin structure.