Summary: A close relationship between complement and prothrombin (thrombin) is indicated by: Both are inactivated at 56°C.Heated plasma and serum become both anti-complementary and anti-clotting.Plasma and serum treated with magnesium hydroxide become markedly inhibitory to complement and thrombin action. The effect is more pronounced in plasma than in serum.The azo dye, Calcomine fast pink 2 BL. Unbl., inhibits both complement and thrombin. Complement and prothrombin (thrombin) are probably not identical for: The conversion of prothrombin to thrombin has no effect on complement.Calcium is essential in the clotting process, but is not needed for the action of complement. Excess calcium, however, will antagonize both thrombin and complement.Aluminum hydroxide will remove or inactivate prothrombin but will not affect thrombin or complement.Heparine is markedly anti-thrombic, but relatively weak in inhibiting complement.Clinically, it was found that a plasma (from a patient with a marked hepatic insufficiency) was strongly anti-complementary, yet showed no disturbance in clotting factors. These findings lead to the conclusion that complement and prothrombin are not identical, but appear to have some similarity in structure which makes them susceptible to certain inhibitory substances such as various azo dyes. Prothrombin probably has a group or special structure that reacts with calcium and thromboplastin. While this structure is essential for the activity of prothrombin, it plays no part in complement activity.