Peptides I and II, two metabolically active peptides from porcine pituitary glands, stimulated antibody production in rabbits. The antiserum obtained had antibodies which interacted with peptides I and II from porcine pituitaries, peptides I and II from human pituitaries, and fraction H prepared by Rudman from hog pituitaries. None of the other pituitary preparations tested showed any interaction with peptides I and II.

All three preparations stimulated the release of free fatty acids from rabbit adipose tissue in vitro. Antiserum inhibited this biological action to a different extent in each of the three preparations. Following an intravenous injection of peptides I and II there was a rapid rise in the plasma concentration of free fatty acids in rabbits. Injection of antiserum together with peptides I and II failed to block the rapid increase in free fatty acids. However, an injection of at least ten times the minimal amount of peptides I and II which raised the plasma free fatty acid concentration produced no increase in a rabbit with a high antibody titer to these peptides.

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