Summary: Colostrum-deprived pigs failed to produce antibody to diphtheria toxoid inoculated intraperitoneally at 3 weeks of age, and responded only slightly to tetanus toxoid administered in the same manner. The immunologic deficiency of colostrum-deprived pigs was overcome by the following procedures, listed in order of decreasing efficiency: a) by suckling of colostrum, b) by administration of diluted hyperimmune serum of swine and horse origin, c) of immune swine serum, d) and of normal serum from older, colostrum-deprived pigs. Removal of specific antibodies from the hyperimmune swine serum resulted in loss of the antibody-stimulating activity of the serum. The results are consistent with the natural selection hypothesis of antibody formation proposed by Jerne.