The plasmid content, antibiotic susceptibilities, and biochemical traits of fecal streptococci isolated from pigs during a tylosin feeding experiment were analyzed. The presence of tylosin in the feed did not select for a particular streptococcal strain or biochemical type but did select for multiple antibiotic resistance. A DNA probe obtained from Escherichia coli and containing a cloned macrolide-lincosamide-streptogramin B (MLS) resistance determinant from a streptococcal strain of human origin hybridized primarily to large plasmids among MLS-resistant streptococci from antibiotic-fed pigs. Patterns of hybridization were similar among MLS-resistant clinical isolates from human infections and isolates from other antibiotic-fed animals. The results of mating experiments indicated that tylosin selected for localization of the MLS determinants to large conjugative plasmids. These data established (1) a strong sequence homology between MLS resistance genes in isolates from humans and those from farm animals and (2) an increased potential for dissemination of MLS resistance as a consequence of tylosin feeding.