Summary: Heparin, capable of reacting with prothrombin or thrombin (20), inactivates complement only in quantities out of all proportion to its anti-coagulative ability. The synthetic anti-coagulants, the sodium salts of polyvinyl sulfuric and cellulose sulfuric acids, are from 75 to 100 times as anti-complementary as heparin, while possessing only 1/15 of the anti-coagulative ability of the purified heparins. Anti-coagulants capable of reacting with calcium, failed to inactivate complement under the conditions employed. Protamine, which probably inhibits coagulation by reacting with the thromboplastic factor (20), likewise required amounts for complement inactivation out of proportion to its anti-coagulative ability. The anti-complementary action of the anti-coagulants may be attributed to their acidic or basic nature, and to their high molecular size. No definite correlation exists between the inactivation of complement and the inhibition of coagulation.