AbstractA basic myotoxic protein was purified fromBothrops asper venom. Like other basicBothropsmyotoxins, myotoxin IV induces acute muscle damage after intramuscular injection in mice and disrupts negatively charged liposomes but not positively charged ones. Furthermore, this protein exerts a weak anticoagulant effect only at concentrations of 40 μg/ml or higher, and is devoid of phospholipase A2activity. Rabbit polyclonal antibodies toB. aspermyotoxin II, a related lysine‐49 isoform, cross‐react strongly with myotoxin IV by enzyme immunoassay, indicating a high degree of antigenic similarity between these two proteins. The fact that both toxins have similar amino acid compositions and amino‐terminal sequences, including leucine‐5 and glutamine‐11, 2 amino acid residues conserved in all lysine‐49 phospholipase A2variants purified, strongly suggests thatB. aspermyotoxin IV is a lysine‐49 phospholipase A2. © 1995