The sensitivity of 332 strains of Serpulina hyodysenteriae isolated in Hungary between 1978 and 1992 was tested against seven chemotherapeutic drugs frequently used for the treatment of swine dysentery, and the changes in the patterns of resistance were also monitored. All the strains remained sensitive to carbadox, with minimum inhibitory concentrations (MIC) of only 0.05 to 0.40 μg/ml at present. The susceptibility of the strains to dimetridazole has gradually decreased, but about half of the strains are still sensitive, with large numbers of ‘moderately sensitive’ strains; the MIC values varied within wide limits (0.1 to 50 μg/ml). Most of the strains were resistant to tylosin, with MIC values from 0.1 to 100 μg/ml. The number of strains resistant to lincomycin has gradually increased, but about half of the strains remain sensitive; the MIC values ranged from 0.2 to 100 μg/ml. Recently, tiamulin has proved the most effective antibiotic, but some resistant strains have already emerged (MIC values 0.05 to 50 μg/ml). Monensin was good for the prevention of swine dysentery, but resistance may evolve quickly; the MIC values ranged from 0.4 to 25 μg/ml. For sedecamycin, the MIC values (6.25 to 100 μg/ml) were much higher than expected.