Embryonic and early posthatching chicks were injected with equivalent amounts of bovine serum albumin and human γ-globulin on the basis of body weight and their subsequent humoral antibody response estimated by quantitative methods. Embryos injected at 11 days of incubation showed, in comparison with saline controls, a slightly lower but statistically nonsignificant antibody response, at 45 days after hatching, and no effect at 100 days. Embryos given a depressive injection on the 16th incubation day showed a statistically significant depression of antibody response at 45 days after hatching, but none at 100 days. Chicks injected at 1 day after hatching showed depression quantitatively similar to that of the 16-day embryos. Chicks at 5 and 9 days of incubation, injected into the yolk sac with large amounts of BSA, showed no effect on their antibody response at 100 days. Intraperitoneal injections into young posthatching chicks were more effective than equal intravenous injections, both as concerns the degree of antibody depression and duration of the depression. Human γ-globulin injected into 16-day embryos, produced a significant depression of the antibody response to bovine serum albumin (cross-depression) at 45 days of age. No cross-depression was discernible at 100 days. Antibody responses of male and female birds, on the basis of mg antibody/ml serum, were not significantly different, nor were body weights of protein-injected birds in comparison with saline controls.