The lamina propria (LP) contains many lymphocytes that are effector cells as well as memory cells of the gut immune system. This compartment was studied in normal and germ-free pigs in the early postnatal period up to 91 d of age. The number of LP lymphocytes nearly doubled between the 1st and 29th d. LP lymphocytes proliferated more in the crypt region than in the villi with a mitotic rate/h comparable to that in nonfollicular compartments of Peyer's patches and lymph nodes. The determination of the subpopulations of LP lymphocytes showed a 10-fold increase in CD22cells between d 1 and 40. About 80% of the LP T cells in 1− and 5-d-old pigs had the unusual CD2+CD4–CD8–phenotype. Ig-positive cells appeared later in the postnatal period than the T cells. On d 1, only a few IgM+cells were observed. In 40-d-old animals, the number of IgA+cells exceeded that of IgM+. Ten times more Ig+cells were detected in the crypt region than in the villi. The germ-free pigs at an age of 49 d had a T cell subset pattern comparable to that of 5-d-old normal animals. (Pediatr Res 29: 237–242, 1991)