Antigen detection for diagnosis of pneumococcal infections has earlier been based on the use of antibodies against capsular antigens. Methods based on the demonstration of C-polysaccharide (PnC) have the advantage of using antibodies against one single species-specific antigen instead of applying a polyvalent mixture of antisera against 83 different capsular antigens. Very little has been done earlier to evaluate the accessibility of the PnC on the surface of pneumococcal cells, particularly cells carrying capsules, and the release of PnC to the environment. We have used a monoclonal anti-PnC antibody in an ELISA inhibition test to demonstrate PnC during growth from four different Streptococcus pneumoniae strains (capsular types 1, 3 and 19F) and a C-mutant strain, reported to carry PnC as a small capsule. Heavily-capsulated types 3 and 19F exposed more PnC on their surface than the type 1 strain, and considerable amounts of PnC were released to the culture medium during growth. The C-mutant strain differed from the other strains in that it exposed less PnC on its surface. The type 1 strain and the C-mutant released approximately the same amount of PnC to the culture medium. Treatment with antibiotics during growth caused a decrease in surface-located PnC but did not significantly affect the amount released. The total accessible PnC in these cultures was quite significant and well above the limit of detection in an ELISA earlier described for the detection of PnC in clinical samples. The results presented here give support to the notion that the demonstration of PnC in clinical samples should provide a good basis for diagnosis of pneumococcal pneumonia.