Eleven species of lactic acid bacteria when grown on a milk-based medium reduced the pH value to < 4.7. The cells of nine of these species inhibited the growth of Escherichia coli on agar in Petri dishes. However, only Streptococcus faecium and Lactobacillus bulgaricus gave significant cell free anti-E. coli activity. Activity against E. coli enterotoxin was also found only in two species. L. bulgaricus and S. faecalis. Part of the activity in the former species was cell free. Further investigation of the cell-free anti-enterotoxic activity from L. bulgaricus showed that it had a low molecular weight probably < 103, was not very stable and was independent of the anti-E. coli activity. Broths containing L. bulgaricus fermented to produce high levels of antienterotoxin were beneficial when added to diets for early weaned pigs. It was inferred that this effect was likely to be caused by the anti-enterotoxic activity.