1. A method of obtaining partially purified products of plasmin degradation of fibrinogen (F.D.P.) is described. The F.D.P. passes through dialysing tubes and does not lose its activity during heating for 15 min at a temperature of 56 degrees C.2. In experiments on isolated organs, F.D.P. in small doses potentiates the contractile action of bradykinin, kallidin, angiotensin, histamine, 5-hydroxy-tryptamine, acetylcholine, adrenaline and noradrenaline. In high concentrations, F.D.P. induces an increase in initial tonus of the smooth muscles and simultaneously lowers the sensitivity of the smooth muscles to the action of the pharmacologically active substances mentioned.3. A similarity in the effect of different concentrations of KCl and different concentrations of F.D.P. on the contractile reaction of the guinea-pig intestine to constant doses of histamine and bradykinin and on the initial tonus of the smooth muscles is observed.4. A hypothesis is presented that F.D.P. in small doses potentiates the contractile action of amines and polypeptides on the smooth muscles and in larger doses increases the initial tonus of these muscles by changing the concentration of the cell electrolytes.