Technical Abstract: The effect of tylosin on erythromycin resistant enterococci was examined on three farms, farm A that used tylosin for growth-promotion, farm B that used tylosin for treatment of disease, and farm C where no tylosin was used. Six hundred and sixty-two enterococci were isolated from gestation, farrowing, suckling, nursery, and finishing swine from these farms. Of these, 59%, 28%, and 2% were resistant to erythromycin (MIC greater than or equal to 8 ug/ml) from farm A, B, and C, respectively. PCR analysis and Southern blotting revealed that 65 isolates chosen from all three farms for further study were positive for ermB, but negative for ermA and ermC. Using Southern blotting, ermB was localized to the chromosome in 56 of the isolates while 9 isolates from farm A and B contained ermB on two similar sized plasmids (12-16 kb). Pulsed-field gel electrophoresis revealed that the isolates were genetically diverse and represented a heterogeneous population of enterococci. This study suggested that although resistance was higher on a farm where tylosin was used as a growth promotant, resistant enterococci also persisted on a farm where no antimicrobials were used.