Nitration studies using tetranitromethane were conducted on the tyrosine residues of cardiotoxins, naja melanoleuca VII1, Naja haje annulifera VII1 and Hemachatus haemachates toxin 12B. Various partially and fully nitrated derivatives were formed. Analysis of the products of nitrating naja melanoleuca VII1 showed that the average relative reactivities of the three tyrosine residues were Tyr-25 greater than Tyr-22 greater than Tyr-51. It was significant that Tyr-51 could be easily modified in both N. melanoleuca VII1 and N. haje annulifera VII1. In contrast, other workers had found Tyr-51 in N. naja atra cardiotoxin to be unreactive towards tetranitromethane except under denaturing conditions. Fully nitrated derivatives of N. melanoleuca VII1 (Tyr-22, -25 and -51), N haje annulifera VII1 (Tyr-22 and -51) and H. haemachates 12B (Tyr-22), prepared under mild reaction conditions, were isolated by ion-exchange and hydrophobic interaction chromatographies. All three derivatives were pure by disc gel electrophoresis at pH 8.9 and amino acid analysis. They were therefore suitable for spectral and biological studies. The results were compared and contrasted to; those of other workers.