Summary: The rates of synthesis and serum concentration of γ-globulin in mice appear to be related to the stimulation provided by intestinal microorganisms. The γ-globulin synthesis rate in normally raised mice is 50 times the synthesis rate in mice with no bacterial flora (germfree), 10 times the synthesis rate in mice with a subnormal bacterial flora, and ¼ the synthesis in mice placed in contact with other strains and species after being reared with a subnormal flora. Serum γ-globulin levels in these mice reflect the rates of γ-globulin synthesis. The mean γ-globulin level in normally reared mice is 35 times greater than in germfree mice. The ability of the mouse to catabolize large amounts of exogenous γ-globulin is not altered in the germfree state.