A new method for the preparation of a relatively pure cephalin, free from lecithin, was developed. Purified cephalin prepared by this method possessed a high degree of coagulative activity. An aqueous solution reacted with CaCl2 in neutral solution to produce a distinct chemical compound of cephalin with Ca, which, when washed free of its reaction product, HCl, was inactive as a coagulant of blood plasma. The cephalin recovered from the Ca compound possessed a high degree of coagulative activity. When an aqueous solution of cephalin was mixed with a solution of serum albumin or pseudoglobulin in the presence of a slight amount of acid (HC1), a globulin-like precipitate was formed whereas in the absence of acid no visible reaction occurred. No precipitation occurred when this amount of acid was added to the cephalin or the protein alone. Similar results were obtained when CaCl2 instead of acid was added to a mixture of cephalin and serum protein. Lecithin, on the other hand, which has no coagulative action, did not give rise to precipitation when mixed with these proteins either in the presence or absence of similar amounts of acid or CaCl2.[long dash]The results indicate that cephalin, like the active lipid fractions isolated from the crude tissue extract and from pure lecithin, coagulates blood plasma and, further, that this action is associated with a chemical reaction of the lipid with ionized Ca salts resulting in the formation of mineral acid.