A Streptococcus pneumoniae latex agglutination (LA) test (Bactigen; Wampole Laboratories, Div. Carter-Wallace, Inc., Cranbury, N.J.) and counterimmunoelectrophoresis (CIE) were compared for the detection of pneumococcal antigen in serum and urine specimens from 68 elderly patients with pneumococcal pneumonia. The cases were categorized according to the presumptive role of S. pneumoniae: definite, putative, questionable (poor score), or questionable (mixed flora). Serum and urine samples were collected on days 1 to 3, 4 to 6, and 7 to 9 of illness and screened in parallel by LA and CIE. LA detected pneumococcal antigen in the serum or urine or both from 31 (46%) of the 68 pneumococcal pneumonia cases compared with 10 (15%) of cases detected by CIE. The highest rates of detection were noted in the 17 definite (bacteremic) cases: 88% by LA and 38% by CIE. The detection rates for both tests were lower in the other nonbacteremic pneumonia categories. Pneumococcal antigen was detected more often in urine specimens than in serum specimens by LA and CIE and was detected in the urine of 92 and 46% of definite cases, respectively, after 7 to 9 days of illness despite antibiotic therapy. Both tests were specific when tested with nonpneumococcal pneumonia cases, but LA detected pneumococcal antigen in two of seven chronic bronchitis cases. This study suggests that LA is as specific and more sensitive than CIE and is useful for detecting antigen in the elderly with proven bacteremic pneumococcal pneumonia. LA is less sensitive for detecting nonbacteremic pneumococcal pneumonia and, therefore, would be of limited value in the care and study of the institutionalized elderly.