A unimolecular film of lecithin spread on an aqueous soln. is converted to lysolecithin by snake venom present in the soln. The reaction is measured by the method of surface potentials, and venom concns. down to 1 part in 4.107 are detectable. The rate of attack decreases greatly on compression of the lecithin molecules in the film. The pH opt. is ca. 7.3. Hemolysis of guinea-pig cells by venom is related to the lecithinase content measured by surface action. Cobra venom is anomalous in that its surface action is inhibited in cones, above 0.0001%.