Exonuclease from Crotalus adamanteus venom has only threonine as N-terminimal amono acid residue. It was examined for its amino acid composition, -SH and S-S groups. It has no free -SH groups and seven S-S bonds. The analysis of the carbohydrate residues in the enzyme proves that it is a glycoprotein. It contains neutral sugars (9.2%), amino sugars (1.9%) and ten sialic acid residues per molecule. The venom exonuclease is a metalloenzyme. This is proven by the existence of Mg2+, Zn2+ and Ca2+ and their specific role in the catalytic reactions. The enzyme contains also triacylglycerols (1.54%) and cholesterol esters (1.43%). The influence of the non-protein moieties of the exonuclease on its catalytic ability has been discussed.