Guinea pig insulin is the only mammalian insulin previously known to be nonneutralizable by antibody to beef insulin (Canad. J. Physiol. Pharm. 43:373, 1965). A second nonneutralizable mammalian insulin, that of the coypu (nutria, or myocastor coypus), has been identified. Coypu pancreas yielded 1.95 U. extractable insulin per gram. An amount of guinea pig antibody to beef insulin sufficient to neutralize twenty-five times as much beef insulin had no effect on the increase in glycogen content produced by pancreatic or serum coypu insulin in the mouse hemidiaphragm in vitro. Both 50 mU. coypu insulin alone and 50 mU. coypu insulin plus 150 mU. antibody to beef insulin when injected into mice produced as many convulsions and lowered the blood glucose to the same mean level as did 50 mU. beef insulin alone (12 mg./100 ml.). Mice injected with 50 mU. beef insulin plus 150 mU. antibody did not convulse and had a mean blood glucose of 108 mg./100 ml. Coypu pancreatic extract cross-reacted with guinea pig antiserum to beef insulin in the passive cutaneous anaphylaxsis test.

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