Colonic lymphoglandular complexes (LGCs) and intestinal mucosa of conventional postweaning pigs were analyzed by flow cytometry and immunohistochemistry using monoclonal antibodies specific for B and T lymphocyte subsets. Lymphocytes were compartmentalized into B (T-independent) and T-dependent (internodular) areas. CD4[superscript]+ cells were more plentiful than CD8[superscript]+ cells. In nodules, CD4[superscript]+ cells were common but CD8[superscript]+ cells few. Ig-containing cells were present between and within nodules. IgA[superscript]+ and IgM[superscript]+ cells outnumbered IgG[superscript]+ cells and all three types were most numerous at the neck of the LGC. IgA[superscript]+ and IgM[superscript]+ lymphoblasts were deep in the complex. IgA and IgM, but not IgG, were present together in some diverticular epithelial cells. Flow cytometric analysis of LGC lymphocytes produced mean subset percentages (and standard deviations): CD2 (pan T) 39.6 (1.5); CD4 29.5 (2.7); CD8 15.8 (1.6); IgA 12.1 (5.2); IgG 5.8 (2.1); IgM 23.5 (1.9); null cells (non-B-non-T) 19; doubly positive for CD4 and CD8 ((CD4 + CD8) - CD2) 6. The mean CD4:CD8 ratio was 1.9 (SD 0.16). High-endothelial venules were identified in the LGC;In intestinal mucosa, IgA[superscript]+ and IgM[superscript]+ cells were common and restricted to pericryptal lamina propria; IgG[superscript]+ cells were uncommon. Intraepithelial B cells were rare. IgA and IgM were present together in crypt epithelial cells; IgG was not in epithelial cells. T cells were quantified and compared as separate populations of intraepithelial lymphocytes (IELs) and lamina propria lymphocytes (LPLs)s. Greater than 95% of IELs were CD8[superscript]+, and IELs were 2-3 times more numerous in small intestine than in colon. CD4[superscript]+ LPLs were twice as numerous as CD8[superscript]+ LPLs. Together, CD4[superscript]+ and CD8[superscript]+ LPLs were 22% more numerous in small intestine than in colon. Both subsets were dispersed randomly in lamina propria;Infrequent and unpredictable uptake by LGC lymphoepithelium occurred after instillation of plain and fluorescein-labeled beads, carbon particles, live and dead Treponema sp. bacteria, cholera toxin, cationized ferritin, and a bovine serum albumin-colloidal gold conjugate into ligated colonic loops of four anesthetized juvenile pigs. By light microscopy, beads, ink, and virulent bacteria were in and beneath the epithelial layer. Bacteria were seen in and beneath epithelial cells by electron microscopy. Alternative experimental methodology might help to confirm enhanced sampling capacity of the LGC versus absorptive mucosa.