AbstractThe effect of phenol and certain phenyl compounds on the different stages of the blood coagulation and fibrinolytic mechanism is investigated. Phenol has a complex action on blood coagulation. In an optimal concentration of 0.3% it produced a definite, though suboptimal platelet factor 3 activity, evolved factor XII, accelerated thrombin–fibrinogen interaction, retarded clot retraction, enhanced the action of streptokinase on plasminogen, and inhibited plasmin. In addition, in the absence of factor XIII and calcium, it increased the solubility of fibrin in urea. Phenyl acetaldehyde, benzyl benzoate, and styrene (phenyl ethylene) accelerated stypven clotting time and thrombin–fibrinogen reaction. Unpolymerized styrene and phenyl ethanol specifically inhibited the action of plasma thromboplastin, formed in a system lacking in factor VIII or IX. A study is also reported on the changes responsible for the stabilization with phenol of brain thromboplastic activity. The action is not purely bactericidal, but is dependent on the presence of brain tissue particles in suspension. The contribution of platelets to plasma thromboplastin formation is examined in the light of these investigations, showing the ability of phosphorus‐free compounds to supply a platelet‐like activity.