Observations were carried out of the interactions between Lactobacillus casei 294/89 and enterotoxigenic Escherichia cofi CCM 612 (OlOl:K99) in uivo. In gnotobiotic lambs, inoculation with enterotoxigenic E. coli (ETEC) resulted in diarrhea with a typical clinical picture and patho-anatomical findings. E. coli adhered to the mucosa of the digestive tract at counts amounting to IO5 per cm2. In these lambs, disturbances of intestinal biochemical processes became evident; proteolytic enzyme activity was significantly reduced. Preventive administration of Lactobacillus cusei inhibited the negative effects of ETEC in gnotobiotic lambs, minimized the clinical signs to those of a very moderate diarrhea in the first 12 h after inoculation and significantly reduced the patho-anatomical findings. Enterotoxigenic E. coli counts decreased by 99.1 and 76% on days 2 and 4 after inoculation respectively, and amounted to lo3 per cm'. The inhibitory effects of L. casei against E. coli were most obvious in the jejunum and ileum. The numbers of adhering E. cofi increased from the duodenum with the length of the gut. ETEC counts in the digestive tract of lambs that had been preventively treated with L. casei amounted to 10' ml-'. It can be assumed that, in addition to competitive exclusion, the inhibitory effect of L. casei upon ETEC adherence was also mediated by a Lactobaciflus-produced substance that inhibited E. coli adhesion to the gut mucosa.