Summary: Very large intraperitoneal injections of bovine serum albumin given within 50 hr after hatching resulted in a reduced antibody response when the animals were subsequently reinjected. The unresponsiveness to the homologous antigen decreased with age since groups of 12- and 22-week-old chickens from the same hatch gave higher antibody titers than the 6-week groups. The degree of responsiveness seems to depend upon the size of dose given at hatching. The antibody response to a heterologous antigen, human γ globulin, was not affected.