Previously, we isolated a high-molecular-weight immunogenic polysaccharide (designated PS) from Pseudomonas aeruginosa immunotype 1 (IT-1). The method which we used was modified to permit the isolation of a similar PS from P. aeruginosa IT-2. This antigen was composed primarily of carbohydrate, had a complex monosaccharide composition, including sugars not found in the lipopolysaccharide, and was nonpyrogenic in rabbits and nontoxic in mice at high doses. This material protected mice from challenges with live homologous cells. P. aeruginosa IT-2 PS gave a line of identity with the O side chain of the lipopolysaccharide, but different from this polysaccharide in molecular weight, chemical composition, and ability to immunize mice actively. Lipopolysaccharide from P. aeruginosa IT-2 contained an immunological determinant not found on P. aeruginosa IT-2 PS, which was detected due to its stability during treatment with dilute alkali. Thus, we recovered a high-molecular-weight PS antigen from P. aeruginosa IT-2, which was serologically identical to the lipopolysaccharide O side chain but was chemically and physically distinct. Also, like P. aeruginosa IT-1 strains, P. aeruginosa IT-2 contains an alkali-stable immunodeterminant on the lipopolysaccharide that may represent a core-like antigen.