One of 2 groups of 15 non-pregnant cows was vaccinated twice 6 weeks apart by the subcutaneous route with a killed adjuvant 45/20 vaccine. At the time of the first vaccination a bull was introduced for a period of 14 weeks. All animals were challenged with 1.5 x 10(7) Brucella abortus organisms given into the conjunctival sac 18 weeks after the second vaccination of the vaccinated group. The largest lesions at the site of vaccination (8 cm diam.) were seen 2 weeks after inoculation. Small lesions were still visible in 8 animals 1 year after vaccination. There was no appreciable serological response to the first vaccination. After the second vaccination some animals gave low positive titres, but all were serologically negative after 10 weeks. After challenge the highest complement fixation and standard agglutination tests were given by animals in the non-vaccinated group. Serum samples from 5 animals (2 vaccinated and 3 not vaccinated) showed a prozone effect at low dilution in the complement fixation test. In the non-vaccinated group 9 of the 15 cows became pregnant, 7 of which had weak calves or aborted and were shown to be infected with Br. abortus. The other 2, of which 1 was serologically positive, reared normal calves. Of the 10 vaccinated cows that became pregnant, 8 reared calves, 1 had a weak calf and the other aborted. These 2 cows and 1 other were infected. Considering the 11 infected pregnant animals 9 or 80 percent, lost their calves. The number of surviving calves was significantly higher in the vaccinated group indicating that the use of this 45/20 vaccine has considerable economic benefit. In each group 3 of the non-pregnant animals were infected.