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Light and electron microscopy with histochemical staining were used to estimate exopolysaccharide production by strains of viridans streptococci recovered from patients with endocarditis. Six strains were selected for study because they represented a wide range of in vitro polysaccharide production. By light microscopy, there was good agreement between three polysaccharide stains (ruthenium red, periodic acid-Schiff and calcifluor white) in the amount of glycocalyx produced, which ranged from minimal (0 to 1+) to maximal amounts (4+). Two strains selected for minimal (strain 1) and maximal (strain 6) in vitro exopolysaccharide production were studied after we used them to experimentally infect cardiac vegetations. Glycocalyx could be demonstrated surrounding organisms in cardiac vegetations, and the relative amounts produced were similar to those seen in vitro. Vegetations formed by glycocalyx-producing strains were also larger than those formed by glycocalyx-deficient strains. Viridans group streptococci which produce exopolysaccharide in vitro also do so within cardiac vegetations. The relationship of exopolysaccharide production to maintenance of endocardial infection is discussed.