The susceptibility of 31 strains of Brucella to various antibiotics was investigated by agar dilution technique. When tested with inocula ranging from 10(4) to 10(6) bacteria/ml, all strains were susceptible to tetracycline, chloramphenicol, streptomycin and gentamicin. More than 25% were resistant to the combination trimethoprim-sulfamethoxazole. With this combination, the maximal antibacterial effect was obtained at a trimethoprim/sulfamethoxazole ratio of 1:19. In 90% of the strains, stable mutants, highly resistant to rifampicin (CMI greater than or equal to 1024 micrograms/ml) were detected. The mutation rate was about 10-7.