Minimum inhibitory concentrations to tetracycline and erythromycin were determined for nineteen isolates of Actinobacillus pleuropneumoniae of Norwegian origin. The isolates were screened for rRNA methylase determinants (Erm genes) and for the tetracycline resistance Tet B determinant, using oligonucleotide probes, polymerase chain reaction and hybridization. Ten isolates (53%) carried the Erm C determinant, two isolates (10%) carried the Erm A determinant, four isolates (21%) carried both the Erm A and the Erm C determinants, and three isolates (16%) carried none of the Erm determinants examined. Eight isolates (45%) carried the Tet B determinant. Selected isolates were shown to transfer the Erm C and Erm A determinants at a frequency of 10(-7)-10(-9) per recipient cell. This is the first description of A. pleuropneumoniae carrying either Erm A, Erm C or/and Tet B determinants.