Summary: The reaction of radioiodine with fragments obtained by papain digestion of rabbit γ-globulin or rabbit antibody has been studied. The γ-globulin digests were examined in the ultracentrifuge and consisted of a single peak with a sedimentation coefficient of 3.5 S. The γ-globulin digests were separated into three major fractions by chromatography on carboxymethyl cellulose columns and were analyzed for radioiodine and protein. In preparations containing whole γ-globulin, Fractions I and II iodinated to about the same extent, whereas Fraction III iodinated to about half the value of I and II. These values were found whether iodination preceded digestion or the reverse. Results with specifically purified antibody (against the p-azophenylarsonate group) differed from the results obtained with whole γ-globulin in three respects: a) The amount of protein in Fraction II was about twice that of Fraction I. b) Fraction II was iodinated at a level 25 to 30% greater than Fraction I. These two findings are interpreted as indicating that Fractions I and II are derived from different antibody molecules rather than representing the two parts of the same antibody molecule. c) Differences were also observed in the relative iodine contents of the three fractions of specifically purified antibody, depending on whether iodination preceded or followed digestion, indicating differences in orientation of the components of the original antibody molecules.