The effect of single and multiple 2 g doses of i.v. cefpirome on pharyngeal and faecal flora was studied in ten male volunteers. There was no effect on pharyngeal flora. After a single dose, cefpirome had no effect on faecal flora but numbers of Escherichia coli were reduced below the detection limit during multiple dose treatment. No strains of Clostridium difficile were selected in this study and only a slight increase in the numbers of Candida spp. were found. Cefpirome, therefore, has little, if any, effect on faecal or pharyngeal flora.