Following myocardial infarction, soluble fibrin in plasma is often elevated as a sign of an activated plasmatic coagulation system. Soluble fibrin in plasma is most pronounced in shock patients under catecholamine administration. With the improvement of the clinical situation the fibrin concentration declines to normal. Following incubation with N-ethylmaleimide an increased production of prostaglandin endoperoxides is observed in the platelets of patients after myocardial infarction compared to normal platelets. Plasma of patients exhibits an endoperoxide-producing effect on normal platelets. The stimulating effect of patient plasma diminishes together with the fall of soluble plasma fibrin. These phenomena may be considered as signs of a relation between the plasmatic and thrombocytic coagulation system.