We have studied the antimicrobial action of metronidazole against Bacteroides fragilis and Escherichia coli both in pure cultures and in combination. Concentrations of metronidazole above the minimal inhibitory concentration (MIC) produced a marked bactericidal effect on B fragilis at six hours. E coli however showed only a slight and transitory decrease in population when exposed to concentrations of metronidazole close to its MIC. Kinetic studies of this drug's effect on mixed cultures of both organisms showed mutual bactericidal synergy between E coli and metronidazole against B fragilis and between B fragilis and metronidazole against E coli. These in vitro findings agree with the results obtained in vivo by other authors.