Lactobacillus casei (L. casei) has been shown to attenuate the severity of experimental colitis. The objective of the present study was to determine whether the effects of L. casei on colitis are related to modulation of leukocyte recruitment into the inflamed intestine. Rats with a colonic segment excluded from fecal transit were surgically prepared. The segment was decontaminated with antibiotics and recolonized with normal flora isolated from inflamed rat colon, associated or not to L. casei. Control and colitic (TNBS-induced) animals were studied. Leukocyte-endothelial cell interactions were characterized in the colonic microcirculation by intravital microscopy, and ICAM-1 and VCAM-1 expression were measured using the radiolabeled antibody technique. In comparison with the non-inflamed colonic segment, induction of colitis by TNBS provoked a marked increase in the number of leukocytes firmly adherent to the venular wall (0.5±0.1 vs. 2.1±0.6 leucocytes/100 µm, p<0.01). Colonization with L. casei significantly reduced the number of adherent leukocytes (1.3±0.4 leucocytes/100 µm; p<0.05), but did not affect the increased rolling interactions associated with the induction of colitis. In comparison with the non-colitic group, induction of colitis was associated with a marked increase in ICAM-1 expression (117±4 vs 180±3, ng antibody/g tissue) that was abrogated when the colitic segment was colonized by L. casei (117±3 ng antibody/g tissue p<0.05). However, L. casei administration did not modify VCAM-1 upregulation in colitic animals. L. casei attenuates leukocyte recruitment observed in experimental colitis induced by TNBS. This effect is possibly related to abrogation of ICAM-1 upregulation.