Wild type (virulent) B. canis organisms are always rough and growth becomes extremely mucoid (M+) after several days of incubation, especially on media pH 7.2. The mucoid growth of virulent (M+) B. canis and the marked tendency for M+ bacteria, and extracted cell wall antigens, to aggregate at acid pH (pH less than 6.5) are markedly diminished in a less mucoid (M-) variant. The M+ and M- Brucella, and their extracted cell wall antigens, also differ in hydrophobic properties; the M+ organisms (and antigens) being the more hydrophobic. Immunoelectrophoresis and immunodiffusion analysis also revealed differences between the two strains. Prolonged survival (greater than 1 year) of B. canis in host tissues, with the production of abortions or epididymitis/orchitis is a principal expression of B. canis virulence. Pathogenicity studies in Beagles have indicated that the M- variant is reduced in virulence, since dogs given high doses of the M- strain by the subcutaneous or conjunctival/oral routes remained asymptomatic. Abortions did not occur in experimentally infected pregnant bitches and there was no epididymitis/orchitis in inoculated males. Nevertheless, M- bacteria persisted in the spleen, prostate tissue of some males, and in the lymph nodes for 3 to 9 postinoculation months. Serological responses of dogs to the M-organisms were generally weaker and less enduring than in dogs infected with the M+ Brucella. Sera from dogs infected with the M- strain generally reacted weakly in agglutination tests, especially when the antigen was B. ovis. The minimum infectious dose of M + B. canis was at least five times less than that of the M - Brucella (conjunctival or subcutaneous routes).(ABSTRACT TRUNCATED AT 250 WORDS)