Resistance to macrolides-lincosamides-streptogramin B (MLS phenotype) appears in almost all streptococcal species isolated from man. Genes coding for MLS resistance are located on plasmids and one MLS resistance transposon has been described. MLS resistance genes have also been found in a large number of plasmid-free strains. Plasmids of 17 to 20 megadaltons (Mdal) that code either for MLS or for both MLS and chloramphenicol resistance are found in streptococci of groups A, B, C, D ( Streptococcus faecalis ) and G. These plasmids have broad host ranges (conjugative intraspecies, interspecies and intergeneric transfer), display similar restriction enzyme patterns and share a considerable degree of homology (78 to 95%). One smaller non-conjugative MLS resistance plasmid has been isolated from Str. sanguis (4.5 Mdal). In group D ( Str. faecalis, Str. faecium ) streptococci, MLS resistance genes are also found on plasmids that carry other antibiotic resistance (tetracycline, chloramphenicol, high-levels of streptomycin and kanamycin). These multi-resistance plasmids are either conjugative or non-conjugative and are of various sizes and molecular species and those that have been tested have narrow host-ranges. The MLS resistance genes of one multi-resistant plasmid, isolated from a strain of Str. faecalis , are located on a transposon of 3·3 Mdal, Tn 917 . Hybridization studies, with MLS determinants as probes, reveal homologies among various plasmid-borne MLS resistance sequences. Elements that are thought to be chromosome-borne mediate multiple antibiotic resistance (including MLS) in streptococci of groups A, B, C, D ( Str. bovis ), F, G, Str. pneumoniae, Str. mitis, Str. sanguis and Str. milleri . Strains harbouring such elements contain no detectable plasmid DNA. In some of the strains these elements are conjugative; their resistance markers transfer en bloc at low frequency and display narrow host ranges. Such elements, from Str.pyogenes and Str. agalactiae , were found to translocate onto various streptococcal haemolysin-bacteriocin plasmids.