The ontogeny of the lymphoid cells in conventional and germ-free mice has been studied. Development is similar in both groups up to the age of 4 weeks. During the first 2 weeks of neonatal life, few cells are present, but the numbers increase rapidly between 2 and 4 weeks of age. The number of cells continues to increase in conventional mice, but remains static in germ-free mice. Exposing adult germ-free mice to a conventional environment results in an increase in the size of the epithelial lymphocyte population to that characteristic of conventional mice. Rearing conventional weanling mice on antibiotic-treated drinking water does not prevent the post-weaning increase in lymphoid cell numbers. The results are interpreted as showing that the lymphoid cell population in the intestinal epithelium is part of the second-level or peripheral lymphoid tissue rather than part of the first-level systems such as the thymus and bursa of Fabricius.