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Molecules of rabbit γ-globulin are split into three nearly equal pieces by papain. Trace labelled with 131 I these three fractions are transmitted by the yolk-sac to the circulation of the 24-day rabbit foetus in significantly different quantities. Fraction III is transmitted nearly as readily as whole γ-globulin, whereas fractions II and I are transmitted only 1/5 and 1/10 as readily, respectively, as fraction III. It is suggested that fraction III, which contains most of the antigenic groups of the original molecules, also has the configuration recognized by the cells as homologous γ-globulin.