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The fetal lamb in utero is able to form large amounts of specific antibody in response to antigenic stimulus as early as the 66th to 70th day of the 150 day gestation period. Among the several antigens employed, the fetal lamb responded earliest, and with the highest titers, to bacteriophage φX. Slightly less effective as an antigen was horse ferritin, while ovalbumin proved to be a weak antigen, especially in younger fetuses. Ineffective in stimulating an antibody response at any time during fetal or early neonatal life were diphtheria toxoid, Salmonella typhosa, and BCG. Thus, it may not be feasible to fix precisely the time of onset of immunologic responsiveness in a species, inasmuch as it appears to differ so greatly from one antigen to another. The quantity of antibody found 10 days after φX immunization was not significantly different in fetuses injected at 60 to 120 days of gestation. The earliest anti-phage antibody produced by the lamb fetus is a macroglobulin sensitive to the action of 2-mercaptoethanol. Only in older fetuses with longer lasting stimuli were appreciable amounts of 7S γ-globulin antibodies formed. The conformity of these observations to theories on the ontogenesis of the immune response is discussed.