In vivo expression of the type 8 capsular polysaccharide (CP) of Staphylococcus aureus was studied by using the Becker strain, the prototype type 8 strain, in a guinea pig model of persistent subcutaneous infection. At 24 hr after infection, supernatants of aspirates from the site of infection contained low levels of CP (median, 0.53 ng/ml; range, less than 0.16-6.30 ng/ml). CP levels increased at day 3 to a geometric mean level of 47 ng/ml, at day 6 to 389 ng/ml, at day 8 to 537 ng/ml, and thereafter persisted in that range. On days 1, 3, and 6, CP was not detectable (less than 0.16 ng/ml) in the serum, but on days 8, 10, or 13, CP was present in 15 (58%) of 26 animals; the median peak level of serum CP among these animals was 2.00 ng/ml (range, 0.31-5.90 ng/ml). These studies document that the type 8 CP of S. aureus is produced and released during a focal, suppurative infection and can be detected in the serum of infected animals.