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Content: If acute cases develop suddenly in an unvaccinated herd, antiserum, if available, may be administered to in-contact pigs. It may be more cost effective to use a long-acting form of penicillin or tetracyclines before implementing a vaccine program. Prevention is best achieved by regular vaccination using killed bacterins or, in some countries, attenuated vaccines prepared by serial passage or strains of low virulence for pigs. The formalin-killed, aluminum-hydroxide-adsorbed bacterin confers an immunity that, in most instances, protects growing pigs from acute disease until they reach market age. An oral vaccine of low virulence is also used. Young breeding stock, including boars, should be vaccinated twice at intervals of 3-5 wk at selection or before entering the herd, and then revaccinated every 6 mo or after each litter. Piglets born to vaccinated sows will be protected for 10-12 wk. Further protection will require vaccination at 12 wk. A booster vaccination 3-5 wk later is recommended. Vaccination of heavily pregnant sows is not advisable.