OPS from 5 g dry weight (or 50 g wet weight) of B. abortus S1119-3 is prepared by adding 400 ml of 2%
(v/v) acetic acid, autoclaving the suspension for 15 minutes at 121°C and removing the cellular debris by
centrifugation at 10,000 g for 10 minutes at 4°C. The supernatant solution is then treated with 20 g of
trichloroacetic acid to precipitate any proteins and nucleic acids. The precipitate is again removed by
centrifugation at 10,000 g for 10 minutes at 4°C. The supernatant fluid is dialised against at least
100 volumes of distilled water and freeze dried