The distributions of tet(M) and conjugative transposons related to Tn1545 were studied by hybridization in 47 clinical isolates of Streptococcus pneumoniae resistant to tetracycline. Resistance to tetracycline was always associated with resistance to minocycline and was due to the presence of the tet(M) gene. Association of tet(M) with int-Tn, the gene encoding the protein required for the movements of Tn1545-like transposons, was found in all but one strain of S. pneumoniae. In contrast, int-Tn was detected in only 2 of 37 strains of Staphylococcus spp. harboring tet(M).