The third most abundant component of black mamba venom, named FS2, was sequenced with the aid of sequenator studies and peptides derived by tryptic and chymotryptic digestion. Cyanogen bromide digests provided extra information to support the proposed structure. This protein is a homologue of the short neurotoxins of snake venom, but is much less toxic. Its structure is quite different from both neurotoxins and the other mamba proteins, called angusticeps types (neurotoxin homologues). Comparison of the known angusticeps-type toxins from mamba venom with mamba neurotoxins and each other leads to proposals that these proteins of low toxicity are inventions of the group of mambas and that three different, as yet unknown, functions will be associated with the three subgroups that are discernable.