Exopolysaccharides (EXPs) of Staphylococcus aureus are associated with virulence in animal models. An EXP from the S. aureus strain Smith diffuse was previously detected in 64.3% of S. aureus clinical isolates. EXP was isolated from culture supernatants of this strain after DNAase, RNAase, phosphodiesterase I and lysostaphin treatment, and was further purified by cation-exchange and molecular-sieve chromatography. Isoelectric focusing revealed a pI of 3.6 for the EXP while the pI of teichoic acid was less than 2.7. Crossed immunoelectrophoresis with homologous Smith diffuse antisera indicated that the EXP contained two immunological components. A major precipitin line persisted after the antisera had been absorbed with the non-EXP-producing variant strain, Smith compact, while the second component was removed. Tandem immunoelectrophoresis also demonstrated that the EXP was distinct from teichoic acid. The EXP contained 2-amino-2-deoxyglucuronic acid, glucose, mannose and galactose. No fatty acids or nucleic acids were present and total protein content was less than 2%. Teichoic acid could not be demonstrated in the EXP, thus further substantiating the immunological studies. S. aureus EXP isolated by the present method can be used for further serological and virulence studies.