Precolostral germfree piglets were perorally immunized with sheep erythrocytes daily, beginning on the 5th day of life. At different intervals the numbers of IgM, IgG and IgA antibody-forming cells were estimated in the spleens, lymph nodes and intestinal mucosa by the haemolytic plaque method. In the beginning of the response IgM antibody-forming cells were detected in intestinal mucosa and mesenteric lymph nodes. IgA plaque-forming cells appeared later and were predominant during the whole period in all organs tested. In all lymphatic organs tested the appearance of IgG antibody-producing cells followed that of IgA plaque-forming cells. The development of immunoglobulins and antibody of different isotypes in intestinal contents and sera was compared with the cellular kinetics found in these young animals after local stimulation.