A sandwich enzyme-linked immunosorbent assay detecting the species-specific pneumococcal C polysaccharide was compared to latex agglutination and a coagglutination test which detected capsular pneumococcal antigens in sputum specimens with regard to specificity and sensitivity. Specimens from 52 patients with clinical and radiological evidence for pneumonia were tested. Twenty-one patients with Streptococcus pneumoniae isolated in sputum and 31 patients with a non-pneumococcal etiology were included. The predictive values for a positive test by enzyme-linked immunosorbent assay was 0.91 and for a negative test 0.97, by latex agglutination 0.90 and 0.91, and by coagglutination 0.84 and 0.85 respectively; these values did not show a statistically significant difference. Whereas agglutination tests are technically more simple and can be performed more rapidly, the enzyme-linked immunosorbent assay has the advantage of detecting pneumococcal C polysaccharide, an antigen common to all pneumococci. Thus it provides an interesting alternative to tests based on serum containing antibodies to all 83 different capsular polysaccharides.