Cl. welchii (type A) toxin contained a lecithinase (optimum pH 7.0-7.6) which decomposed lecithin into phosphocholine and a diglyceride. This lecithinase was probably identical with the specific [alpha]-toxin which was the lethal, hemolytic and necrotic substance predominant in type A culture filtrates. The lecithinase was relatively heat-stable but was readily inactivated by surface denaturation and by Na dodecyl sulphate. The enzyme was activated by Ca++ and inhibited by fluoride, citrate and phosphate, and was specifically inhibited by Cl. welchii (type A) antitoxic sera. Methods were described for the estimation of the lecithinase activity of toxins and the corresponding anti-lecithinase activity of Cl. welchii antitoxic sera. The anti-lecithinase activity of Cl. welchii (type A) antitoxic sera ran parallel with the animal protection titre (L+) and was a fair measure of the [alpha]-antitoxin. The rate and amt. of hydrolysis by the lecithinase, alone and in the presence of antitoxin, were greatly affected by the conc. of Ca++ present. The significance of these results was discussed in relation to the estimation of the hemolytic titre of Cl. welchii toxins.