Attempts to demonstrate Streptococcus pneumoniae, or its soluble type-specific polysaccharide antigen were made on specimens from 205 patients with acute community-acquired pneumonia by (i) blood, nasopharynx and sputum culture, (ii) counterimmunoelectrophoresis (CIE) of serum, urine and sputum specimens utilizing omni-, pool- and type-specific rabbit antisera, and (iii) microscopy of gram-stained sputum smears. Pneumococci could be cultured and/or pneumococcal antigen detected in specimens from 93 (45%) of the patients with the following frequencies in samples obtained before (and after) start of antimicrobial treatment: culture from blood 25% (6%), nasopharynx 59% (18%) and sputum 80% (13%), CIE on serum 3% (9%), urine 8% (18%), urine 20X concentrated 18% (24%), and sputum 56% (56%). Microscopy of sputum smears revealed pneumococci in 65% (17%) of the 93 patients. The value of cultures from blood and sputum was most evident when the samples were obtained early in the course of the disease. With nasopharynx culture and CIE on sputum the number of positive results was doubled if the sampling was repeated. For rapid diagnosis direct Gram stain and CIE on sputum were about equally effective and also strikingly complementary, alone diagnosing 42% and 48% of the patients, respectively, but together detecting 65% of them. Gram stain was most rewarding on early pretreatment samples and CIE on post-treatment samples.