For the purpose of developing and improving vaccines to protect cattle against bovine genital campylobacteriosis, the interaction of vaccine dose, strain(s) of Campylobacter fetus, and type of adjuvant was studied in the pregnant guinea pig model. Five strains of C fetus were used in the preparation of 20 monovalent vaccines containing various doses of bacteria (0.1 mg to 20 mg, dry weight) in Freund incomplete adjuvant, Freund complete adjuvant, or aluminum hydroxide gel adjuvant. Vaccinal protection against homologous or heterologous challenge exposure with viable C fetus strains was measured in terms of infection, abortion, and serum antibody titers. Major emphasis in the study was placed on vaccines containing highly virulent C fetus subsp venerealis strains (1289 and 17761) that possess most of the heat-labile K antigens that have been identified. Vaccines incorporating strain 1289 provided greater protection against abortion than did strain 17761 vaccines (difference, 29%; P less than 0.05). A positive (P = 0.61) correlation was seen between protection from abortion and strain 1289 serum antibody titer. When the titer was greater than or equal to 4, protection was greater than 80%, whereas abortion was associated with a serum antibody titer less than or equal to 2. In an experiment comparing 2-mg vs 20-mg doses of strain 17761 prepared in aluminum hydroxide gel adjuvant, protection against challenge exposure with the heterologous strain 23 was significantly (P less than 0.05) lower than that against challenge exposure with the homologous strain at the 2-mg dose.(ABSTRACT TRUNCATED AT 250 WORDS)