Capsular polysaccharide was detected in the serum in 19 of 46 patients with pneumococcal pneumonia. Antigenemia was strongly associated with bacteremia and with infection by low-numbered serotypes. During antibiotic therapy, the concentration of polysaccharide in the circulation declined progressively, but circulating antigen remained detectable in two thirds of cases for 2 weeks or longer. The development of measurable type-specific antibody was delayed in patients with antigenemia. It is not known whether this delay was due to diminished antibody production or to neutralization of antibody by circulating antigen. Despite effective antibiotic therapy many patients with antigenemia had a severe and protracted illness; this may have been related to diminished availability of antibody early in the infection.