The effect of the inoculation of three Lactobacillus plantarum strains upon lactic, acetic, acetoacetic and propionic acid levels in the mucosal film (F) and the jejunal and ileal contents (O) has been investigated in gnotobiotic pigs. In the jejunum of the inoculated animals, the mucosal film revealed significantly increased levels of lactic, propionic and acetoacetic acids when compared to the contents (25.3 vs. 10.8 mmol.l-1, 18.5 vs. 5 mmol.l-1 and 29.7 vs. 11.2 mmol.l-1, respectively) as well as insignificantly increased acetic acid levels (11.0 vs. 5.8 mmol.l-1). In the ileum of gnotobiotic pigs, propionic acid levels of the mucosal film were significantly higher than those of the contents (21.2 vs. 9.5 mmol.l-1, p < 0.05). In comparison to the contents, the increased lactic, acetic and acetoacetic acid levels in the film proved to be insignificant. The above results suggest that the significantly increased levels of the Lactobacilli-produced organic acids may present an efficient barrier inhibiting the adherence of digestive tract pathogens to the intestinal mucosa.