Development of fecal Lactobacillus and coliform in healthy newborn pigs during the first 48 h after birth was studied. Lactobacilli were detected (104 per g) in the feces of newborn pigs as early as 4 h after birth and colifroms by 8 h (105 per g). By 24 h the two types were present in near equal numbers (104 to 105/g). A frozen concentrate of a human isolate of Lactobacillus lactis was fed to piglet litters (8 to 10 animals per litter) from the time of their birth. Bottle feeding resulted in reduced fecal coliforms in nursing pigs but lactobacilli were not increased in number. After 54 days of treatment, the Lactobacillus to coliform ratio (L/C) was 1280:1; in control pigs not fed lactobacilli, the ratio was 2: 1. A continued suppression of coliforms was observed for 30 days after treatment was discontinued. The influence of Lactobacillus on the bacterial flora of the gastrointestinal tract was studied. With scouring pigs, enteropathogenic Escherichia coli (EEC) were present in larger numbers in tissue homogenates of the tract than in the lumen. The virulence of the EEC found prosent was confirmed by experimental infection in pigs. In control, nonscouring pigs only non-EEC were isolated from tissue sections. In Lactobacillus-fed pigs, E. coli was reduced to low numbers; also, the few E. coli observed were non-enteropathogenic. There were higher numbers of lactobacilli in tissue sections of Lactobacillus-fed pigs than in control and scouring pigs. The lactobacilli isolated from tissue homogenates of the treated animals resembled biochemically and serologically (fluorescent antibody staining) the Lactobacillus which was fed. Histological studies were done to observe the bacteria in frozen sections of washed intestine obtained from Lactobacillus-fed pigs; staining revealed large numbers of gram-positive bacilli. On the other hand, control pigs which died of scouring revealed many coliform types present. Pigs in groups receiving colostrum and lactobacilli did well; no evidence of diarrhea was seen and many lactobacilli were observed in tissue throughout the small intestine. Even after the challenge with EEC serotype 09:K:NM, these two groups of pigs did not show any signs of disease and few coliform types (cocco-bacillary forms) were observed. Pigs not receiving colostrum but only lactobacilli did not scour before challenge and many lactobacilli were present in tissue from the small intestine. However, 72 h after challenge these latter animals revealed symptons of diarrhea and coliforms were seen in the small intestine tissue in addition to lactobacilli.