The jejunum and ileum of 5 day old and adult normal pigs and of 45 day old germ free pigs were used to study the lymphocyte pools in the epithelium and lamina propria by sequential treatments with EDTA, four hours, and 12 hours of collagenase treatment. In adult animals the incubation of the jejunal wall with EDTA resulted in mean (SD) 26.8 (10.9) x 10(6) intraepithelial lymphocytes per g of tissue. The ileal wall gave lower cell yields. After complete digestion of the lamina propria by collagenase a further yield of 35.2 (10.2) x 10(6)/g lymphocytes was achieved. The separation of the gut wall from 5 day old pigs resulted in a 10-fold lower total lymphocyte yield, and the tissue was totally digested after four hours of collagenase treatment. Many eosinophils and mast cells were found in the suspensions from adult animal tissues after the collagenase treatment; 4.7 x 10(6)/g and 4.8 x 10(6)/g, respectively. The suspensions after 12 hour collagenase incubation contained up to 30% plasma cells. Almost all cells isolated by EDTA incubation were CD8+ T cells. After collagenase incubation CD4+ and CD8+ T lymphocytes were found in all animal groups, and in adult animals up to 20% surface Ig+ cells were harvested. When the incorporation of the thymidine analogue bromodesoxyuridine was used to study the lymphocyte production in vivo 3 to 7% lymphocytes in the epithelium were labelled 24 hours later (lamina propria T lymphocytes about 1%). In this study lymphoid as well as non-lymphoid cells have been analysed in mucosal cell suspensions. The absolute cell yield per gram of mucosal tissue is a basis to estimate the pool sizes of intraepithelial and lamina propria lymphocytes.