The tetracycline (Tet) determinants, which encode resistance either to tetracyclines without minocycline (Tcr) or to tetracyclines including minocycline (Tcr-Mnr), of 30 wild-type clinical isolates of Enterococcus faecium were identified and localized. The Tet determinants were transferred by conjugation into a plasmid-free Enterococcus faecalis recipient at frequencies of 10(-6) to 10(-9) transconjugants per donor, as follows: Tcr, 6 strains; Tcr-Mnr, 14 strains; both Tcr and Tcr-Mnr, 6 strains; no detectable transfer, 4 strains. Classes L (Tcr phenotype) and M and O (Tcr-Mnr phenotype) of the Tet determinants were identified by DNA-DNA hybridization experiments. The Tet L determinant was plasmid-borne in 18 strains and was chromosomal in 2 strains. Tet M was chromosomal in 27 strains and plasmid-borne (pIP1534) in 1 strain; pIP1534 also carried Tet L. Tet M was located on Tn916-like elements in 22 strains and on a Tn916-modified element in 1 strain. Tet O was detected in only one strain in which it was plasmid-borne. Both Tet L and Tet M determinants were carried by 19 strains. One strain carried, in addition to chromosomal nonconjugative Tet L and Tet M determinants, a conjugative Tcr-Mnr marker which did not correspond to any Tet determinant tested in this study. These results attest to the genetic complexity of tetracycline resistance in E. faecium strains.