The correlation of aminopeptidase activity in the homogenate and perienteric fluid from Ascaris suum to the concentration of the studied fluids was assessed. The obtained non-linear correlations were interpreted as a result of the presence of the inhibitor. In further experiments, it was demonstrated that aminopeptidase inhibitor is present in the homogenate and perienteric fluid from A. suum. The effect of this inhibitor was studied using different substrates. The effects of the inhibitor on aminopeptidase activity of A. suum homogenate and on that of rabbit serum were studied. In other experiments it was found that an excess of the substrate impairs the effect of the inhibitor.