The complement-enhancing property of bovine carboxypeptidase A is lost when the enzyme is treated with acetylimidazole. Bovine carboxypeptidase B is as effective in enhancing guinea pig complement as carboxypeptidase A when used at five times the molar concentration as the latter. These data suggest that the peptidase, and not the esterase, activity of carboxypeptidase A is associated with the complement-enhancing property. Bovine procarboxypeptidase A, inert in immune hemolysis, will, when treated with trypsin, enhance guinea pig complement.

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A preliminary report of this work was given before the Federation of American Societies for Experimental Biology in April 1965, at Atlantic City, New Jersey. This work was supported by Research Grant E3757 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland.

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