In this study we report the isolation and purification of the capsular polysaccharide elaborated by Staphylococcus aureus SAl mucoid. The capsule was isolated from bacterial extracts and culture supernatants by a series of ethanol precipitations and enzyme digestions, followed by ion-exchange chromatography. Teichoic acid contamination was eliminated by oxidation with sodium metaperiodate, and the final product eluted in the void volume of a Sephacryl S-300 column. The purified capsular polysaccharide was analyzed by gas-liquid chromatography-mass spectroscopy, 13C and 'H nuclear magnetic resonance, amino acid analysis, immunelectrophoresis, and numerous biochemical assays. The major constituents of the capsule were 2-acetamido-2-deoxy-a-galacturonic acid (4-0 linked), 2-acetamido-2-deoxy-a-fucose (3-0 linked), and taurine. The polysaccharide also contained O-acetyl groups which were removed by mild alkaline hydrolysis. Serologically and biochemically, the capsule from strain SAl mucoid appeared very similar to that produced by strain M. Purified capsular polysaccharide was immunogenic in both rabbits and mice. The optimal immunizing dose in mice was 0.1 ,ig of purified capsular polysaccharide administered intraperitoneally. SAl mucoid resisted opsonophagocytic killing by human leukocytes and complement. However, antibodies raised to the purified capsular polysaccharide neutralized the antiphagocytic effect of the capsule.