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Streptococcus suis causes meningitis, sepsis, and other serious infections in newborn and young pigs and in adult humans. The Gal alpha 1-4Gal-binding adhesin of S. suis was purified to homogeneity by ultrasonic treatment, fractional ammonium sulfate precipitation, and preparative polyacrylamide gel electrophoresis. Pigeon ovomucoid, a glycoprotein with Gal alpha 1-4Gal terminals, was used to detect the adhesin by blotting. The purified adhesin appeared as single band of an apparent size of 18 kDa and of a pI of 6.4; no disulfide bridges were present. The amount of adhesin as revealed by pigeon ovomucoid binding correlated with the hemagglutination activity of different S. suis strains. The purified adhesin bound to latex particles induced hemagglutination which was specifically inhibited with the same inhibitors as hemagglutination by the intact bacteria, thus demonstrating that the purified protein was the Gal alpha 1-4Gal-recognizing adhesin of S. suis. Two adhesin variants (PN and PO) with differing Gal alpha 1-4Gal binding specificity had the similar electrophoretic mobilities and the same N-terminal peptide sequences, indicating that they were closely related. This represents the first isolation of an adhesin with well-defined cell surface carbohydrate binding activity from Gram-positive bacteria associated with meningitis.