The development of the intestinal B-cell compartment in C3H/He mice was studied as a function of age by quantification of the number of intestinal immunoglobulin-secreting cells (Ig-SC). Before and at weaning, the number of Ig-SC in the small intestine (SI) was below 10(3) Ig-SC per SI. During the first few weeks after weaning, this number rose steeply and continued to rise until the mice were about 48 weeks old, when a maximum of more than 25 x 10(6) Ig-SC per SI was found. After 1 year of age the number of Ig-SC decreased. At all ages, the great majority of Ig-SC in the SI produced IgA. The increase of the number of IgA-SC in the SI after weaning is reflected in the amount of IgA in intestinal secretions measured by ELISA. The number of Ig-SC in the SI was compared with the number of Ig-SC found in spleen, bone marrow, Peyer's patches and mesenteric lymph nodes. Striking differences were observed between the SI and the other organs tested in total number, isotype distribution and kinetics of the increase of Ig-SC during ontogeny. These differences are discussed in relation to the regulation of the immune response in the SI and the migration patterns of lymphocytes in mucosal tissues.