The transfer of ermA and ermC genes, the two most common resistance determinants of erythromycin resistance, was studied with Luria-Bertani broth in the absence of additional Ca(2+) or Mg(2+) ions. Fifteen human and five poultry isolates of Staphylococcus aureus, which were resistant to erythromycin but carried different genetic markers for erythromycin resistance, were used for conjugation. Since both the donors (Amp(s)-Tet(r)) and recipients (Amp(r)-Tet(s)) were resistant to erythromycin, the transconjugants were initially picked up as ampicillin- and tetracycline-resistant colonies. The resistance transfer mechanisms of the chromosomally located erythromycin rRNA methylase gene ermA and the plasmid-borne ermC gene were monitored by a multiplex PCR and gene-specific internal probing assay. Four groups of transconjugants, based upon the transfer of the ermA and/or ermC gene, were distinguished from each other by the use of this method. Selective antibiotic screening revealed only one type of transconjugant that was resistant to ampicillin and tetracycline. A high frequency of transfer (4.5 x 10(-3)) was observed in all of the 23 transconjugants obtained, and the direction of tetracycline and erythromycin resistance marker transfer was determined to be from poultry to clinical isolates. The transfers of the ermA and ermC genes were via transposition and transformation, respectively.