A comparison was made of the methods of coagglutination (using sensitized staphylococci) and counterimmunoelectrophoresis with regard to (i) sensitivity in detecting capsular polysaccharides from 14 different types of pneumococci, and (ii) sensitivity and specificity in detecting pneumococcal antigen in sputum samples from patients with pneumonia. Counterimmunoelectrophoresis was 20-150 times less sensitive than coagglutination in detecting purified type 7F and 14 polysaccharide preparations, and was as sensitive as or slightly more sensitive than coagglutination in detecting the other 12 polysaccharide types. Of 331 sputum samples examined, 94 were antigen positive, 87 by coagglutination and 76 by counterimmunoelectrophoresis; strong reactions were obtained with 85 and 63 samples, respectively. Moreover, with counterimmunoelectrophoresis 24 samples gave rise to probably non-specific precipitates, while with coagglutination either a strong positive or an unequivocally negative result was obtained in all but six samples. It is concluded that coagglutination is not only simpler and faster than counterimmunoelectrophoresis, but also appears to be more specific and sensitive for the detection of pneumococcal antigen in sputum samples.