A mobile genetic element, designated Tn1545, was detected in the chromosome of Streptococcus pneumoniae BM4200, a clinical isolate multiply resistant to antibiotics. The 25.3 kb element conferred resistance to kanamycin and structurally related aminoglycosides by synthesis of a 3'-aminoglycoside phosphotransferase type III (aphA-3), to macrolide-lincosamide-streptogramin B-type antibiotics (ermAM), and to tetracycline (tetM). Tn1545 was self-transferable to a recombination deficient S. faecalis strain where it was able to transpose to various sites, induce insertional mutations and was apparently cleanly excised. The element also conjugated to and transposed to the chromosome of S. faecalis, S. lactis, S. diacetylactis, S. cremoris, S. sanguis, Staphylococcus aureus, and Listeria monocytogenes. The properties of the conjugative transposon Tn1545 could account for the sudden emergence, rapid dissemination, and stabilisation of multiple resistance to antibiotics in S. pneumoniae in the absence of plasmids.