Snake venoms have provided a number of novel ligands with therapeutic potential. We have constructed a partial cDNA library from the mRNA of Ophiophagus hannah (king cobra) venom gland tissue and identified five new genes encoding proteins belonging to the three-finger toxin family of snake venom proteins. We have isolated and characterized one of these -sheet containing proteins with a mass of 7012.43 ± 0.91 Da from the venom. The protein was nonlethal up to a dose of 10 mg/kg when injected intraperitoneally into Swiss albino mice. However, it induces labored breathing and death at a dose of 100 mg/kg. It does not show any hemolytic or anticoagulant activity. It caused a dose-dependent decrease of heart rate in vivo (anesthetized Sprague-Dawley rats) and also ex vivo (Langendorff isolated rat heart). This is in contrast to classical cardiotoxins from snake venom that increase the heart rate in animals. Radioligand displacement studies showed that this protein targets -adrenergic receptors with a binding affinity (K_i) of 5.3 and 2.3 µM toward ₁ and ₂ subtypes, respectively, to bring about its effect, and hence, it was named as -cardiotoxin. This is the first report of a natural exogenous beta-blocker.--Rajagopalan, N., Pung, Y. F., Zhu, Y. Z., Wong, P. T. H., Kumar, P. P., Kini, R. M. -Cardiotoxin: a new three-finger toxin from Ophiophagus hannah (king cobra) venom with beta-blocker activity.