Since the hæmolysins of the several venoms respond differently to photodynamic action, they may be regarded as possessing different chemical constitutions. As regards stability, cobra hæmolysin ranks first, daboia second, and Crotalus third. The toxicity of all the venoms is more or less diminished by eosin and erythrosin in sunlight. This reduction in toxicity depends upon chemical changes, of more or less profound nature, taking place in certain of the active principles of the venom. The more stabile the predominant active principles the less the reduction in toxicity, and vice versa. Venom-neurotoxins are highly resistant to photodynamic action, venom-hæmolysins are less resistant, while the hæmorrhagin and thrombokinase of Crotalus and daboia venoms exhibit weak powers of resistance to their action. Hence it follows that while cobra venom remained almost unaltered, rattlesnake and daboia venoms were greatly reduced in toxicity when mixed with the fluorescent dyes and exposed to sunlight. There is an interesting parallel between the action of eosin and erythrosin upon the different venoms and their reactions to other injurious agencies. For example, the hæmolysins of cobra and daboia venoms are more heat resistant than the hæmolysin of Crotalus venom, and the former are less injured by the dyes than the latter. The neurotoxin of the former venoms is also more heat stabile than that of the rattlesnake, and the same relative degree of resistance holds for this substance and the anilines. Just as the hæmorrhagin of rattlesnake venom and the thrombokinase of daboia venom are destroyed by a temperature of 75° C., so are they readily inactivated by the photo dynamic substances employed. The globulin-precipitating and blood corpuscle-protecting principle of cobra venom is relatively thermostabile and in contradistinction to the immunity-precipitins it is also unaffected by eosin and erythrosin. This study of the action of photodynamic substances upon snake venoms serves again to bring out the fact of their highly complex nature, and while enlarging somewhat the field in which photodynamic activity is known to operate, it also proves that this form of destructive activity is affected by the same conditions of resistance as confront the action of the usual physical and chemical agents.