The question as to whether or not the neutralization of prolan by antiprolan is subject to quantitative laws is of interest since it permits of certain conclusions in regard to the mechanism of this reaction. We know that fermentive processes are not subject to quantitative laws. On the other hand, adsorptive processes (e. g., the toxin-antitoxin reaction) are so, subject to the deviation known as Danysz's phenomenon. This phenomenon means: if a certain quantity of toxin is added to a certain quantity of antitoxin the mixture is more toxic if the toxin is added by a fractional method than if it were added at one dose. If, however, the reverse process is followed and the antitoxin is added to the toxin by this method the toxicity of the mixture is not greater than if the antitoxin were added all at once. Bordet described the same phenomenon in connection with hemolysins and Craw with agglutinins. Following the technic indicated by Zondek and Sulman we investigated whether Danysz's effect is also apparent in connection with the prolan-antiprolan reaction. In the first series of experiments we added prolan to antiprolan by the fractional method, in the second we followed the reverse method, adding antiprolan to prolan. The experimental order was chosen so that each time 10 RU of prolan or 10 PAU of antiprolan were dissolved in 0.5 cc of water. The prolan powder had the titer of 1 mg = 100 RU, the antiprolan powder that of 1 mg = 1 PAU. We added the substance at intervals of 2 hours. During the whole experimental period, lasting 8 hours, the substances were kept in the incubator. Table I shows that the neutralizing effect of prolan and antiprolan is subject to strictly quantitative laws.