Seventy tetracycline-resistant Streptococcus pneumoniae were tested for the presence of tetracycline resistance genes, tetM and tetO, using a polymerase chain reaction (PCR) assay and DNA-DNA hybridization. Seven isolates representing five serotypes (12, 22, 6A, 19F and 23) carried the tetO gene. Five of the isolates were genetically unrelated as judged using pulsed field gel electrophoresis (PFGE) analysis. Two 19F isolates came from the same patient, carried both tetM and tetO genes and had the same PFGE pattern. The other 63 isolates carried only the tetM gene. DNA sequences from three of the tetO-carrying isolates were determined; they showed 91-95% nucleotide sequence identity over 300 nucleotides, and 93-95% amino acid sequence identity over 100 amino acids. The isolates carrying both tetO and tetM genes could transfer the tetM gene into both Enterococcus faecalis and S. pneumoniae recipients, but not the tetO gene. There was no detectable transfer of the tetO gene, by conjugation, from the other five isolates.