Of 17 pigs vaccinated with Streptococcus suis type-2 capsular polysaccharide plus Freund's Incomplete Adjuvant, all developed opsonizing antibody against Str. suis type 2. Of 14 pigs vaccinated with type-2 polysaccharide alone, 4 (possibly 6) developed opsonizing antibody. It is possible that some pigs vaccinated with polysaccharide plus Freund's Incomplete Adjuvant developed opsonizing antibody in response to a ‘booster’ injection of polysaccharide alone. Of 21 unvaccinated control pigs, late bleeding from 3 showed opsonizing activity against Str. suis type 2.