The activities of four enzymes (beta-glucuronidase, nitrate reductase and nitroreductase) in selected intestinal bacteria (Escherichia coli, Clostridium sp., Streptococcus sp., Bacteroides sp. and Lactobacillus salivarius) were measured after growth in vitro and in vivo. The five strains differed in their activities with Clostridium sp. being the most active for beta-glucosidase, beta-glucuronidase and nitroreductase, and E. coli the most active producer of nitrate reductase. Enzyme activity in vivo tended to be higher than in vitro but there were instances where the comparative activities were reversed.