The inhibitory activity of seven strains of faecal streptococci against 34 strains of Clostridium difficile was examined in vitro after growth of the streptococci for 24 and 48 h. All strains of C. difficile were inhibited at 48 h but at 24 h the inhibition was variable. Streptococcus faecium, a group D streptococcus and an ungroupable streptococcus exhibited the most striking inhibitory activity. Lowering of pH of the medium occurred at the site of inhibition, but the pH change alone did not explain the inhibition of C. difficile. This antagonism may be related in vivo to the resistance to intestinal colonisation by C. difficile exhibited by the normal bowel flora, and in vitro to failures to isolate C. difficile from faecal specimens.