Monensin and tylosin have activity against gram-positive bacteria, and it has been theorized that their effects on the intestinal environment may promote proliferation of gram-negative bacteria such as Escherichia coli. Effects of these antibiotics on the shedding of E. coli O157:H7 were studied in a feedlot environment, using 32 finishing steers. A diet containing 85% barley grain, 10% barley silage, and 5% supplement was amended with 33 ppm monensin, 11 ppm tylosin, both of these additives, or no additives (control). All steers were orally inoculated with 10(10) CFU of a mixture of four strains of nalidixic acid-resistant E. coli O157:H7. Fecal (grab), oral (mouth swab) and water, water-water bowl interface, feed, and pen floor fecal pat samples were collected weekly for 12 weeks. Prevalence of E. coli O157:H7-positive fecal grab samples did not differ (P = 0.26) among treatments, nor did the rate (P = 0.81) or duration (P = 0.85) of shedding of the organism. Fecal grab samples were positive for E. coli O157:H7 more frequently (P < 0.001) than were oral swabs. More (P = 0.02) E. coli O157:H7-positive oral swabs were recovered from the tylosin group than from controls. E. coli O157:H7 was not detected in any of 47 water samples, but was present in 1 of 47 water bowl swabs, 7 of 48 feed samples, and 36 of 48 fecal pats. Pulsed-field gel electrophoresis suggested that differences existed among inoculated strains in their ability to persist in animals and in the environment. However, this study revealed no evidence that dietary inclusion of monensin or tylosin, alone or in combination, increased fecal shedding of E. coli O157:H7 or its persistence in the environment.