1 - Protamine was incubated "in vitro", with human plasma and serum, and the initial degradation products (1-4 hours at 30 degrees C) were characterized by paper chromatographic and paper electrophoretic methods. The analysis revealed the presence of the arginine and also, in low quantities, that of a supplementary compound, an arginine peptide. Comparative experiences with the same substrate revealed that carboxypeptidase B liberated exclusively the C-terminal arginine, plasmin exclusively peptides of arginine, and trypsin, besides arginine peptides, minutes quantities of free arginine. 2 - The method used for quantitative determination of arginine liberated by the action of plasma and serum is based on coprecipitation of residual and partly degradated protamine with the plasma proteins by addition of methanol-acetone. The arginine recovered from supernatant is determined with the Sakagushi reagent. 3 - We could thus demonstrate that the protaminase activity of human plasma and serum in greatly enhanced by cobalt and is inhibited by cadmium and EDTA. Trypsin inhibitor of Künitz and PMSF have little effect. These characters are those of the human carboxypeptidase N of Erdös and al. 4 - The comparison of protamines from different laboratories did not reveal essential differences between protamine sulfate and protamine chloride as some authors have claimed. 5 - The results of this investigation suggest that the "in vitro" enzymatic action of human plasma and serum on protamine is mostly due, at least during the first hours of incubation, to the carboxypeptidase N, described by Erdös and al. A low enzymatic additional action on protamine was observed. This action of proteasic character was not inhibited by the trypsin inhibitor of Künitz.