Eighty-four anaerobic fecal isolates obtained from five healthy volunteers were tested for their ability to inhibit in vitro growth of eight species of Enterobacteriaceae, four species of faculative gram-positive cocci, and Pseudomonas aeruginosa. Forty-nine of the 84 anaerobic isolates (58%) inhibited the growth of at least one indicator bacterium. Isolates of Bacteroides and Bifidobacterium spp. were most consistently inhibitory. Anaerobic cocci and clostridia were infrequently inhibitory; eubacteria showed no inhibitory activity. Serratia marcescens was the indicator most often inhibited; 54% of all anaerobic isolates tested, all of nine Bifidobacterium isolates and 33 of 43 Bacteroides isolates inhibited this organism. No anaerobes inhibited the growth of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Citrobacter freundii, Citrobacter diversus or Streptococcus faecalis.