Recent data has shown differences between colon and small intestine (SI) intraepithelial lymphocytes (IEL) eg. most SI IEL are CD8+ T cells whereas most colon IEL are CD4+ T cells. This study was done to define differences in homing receptors and activation and memory markers between colon vs. SI IEL and lamina propria lymphocytes (LPL) and draining lymph nodes. CD44, CD45RB, L-selecfin, CD69, CD2, Thy 1, beta 7 integrin expression were measured by flow cytometry on spleen, MLN, caudal node, cecal follicle, colon and SI IEL and LPL of C3H/HeJ mice. Major differences were found between SI and colon IEL In SI only 9% were CD4+ cells vs. 46% in colon. CD4+ SI IEL had a more activated phenotype than their colonie counterparts.: 60 vs. 40% were CD44+, 45 vs. 70% were CD45ni, 0 vs. 30% were L-selectin+, 18 vs. 96% were CD2+, 97 vs. 75% were beta 7 integrin, but there were no significant differences for Thy1 and CD69. In the LPL population CD 4+ cells were more activated in SI as well: 26 vs. 77% were CD45RBh', 34 vs. 96% were CD2+. CD4+ cells of the cecal follicle were more activated than CD4+ cells in caudal node and MLN (cecal follicle vs. MLN/caudal node): 20 vs. 58% were L-selectin+, 40 vs. 62% were CD 45RBhi. Lymphocytes from SI and cecal follicle have a higher ratio of activated cells than colon IEL, LPL, MLN and caudal node. Despite the enormous bacterial antigenic load the colon IEL and LPL did not differ much from soleen. MLN and caudal node.