The in vitro activity of rosamicin and erythromycin was compared at various pH values against 311 strains of bacteria representing common urinary tract pathogens. Alkalinization of the media consistently and significantly increased the antibacterial activity of rosamicin against all of the organisms tested. This was also true for erythromycin except when tested against strains of Proteus. At pH 8, rosamicin was two- to sixfold more active than erythromycin against Enterobacteriaceae. The activity of both antibiotics against Pseudomonas aeruginosa was very similar when tested at pH 8. Erythromycin was twice as active as rosamicin at pH 8 against group D streptococci. The activity of both antibiotics was bacteriostatic and inoculum size dependent, regardless of the organism tested or the pH of the test media. The greater activity of rosamicin against Enterobacteriaceae warrants clinical investigation.