Latex particle agglutination for Streptococcus pneumoniae was evaluated in 76 patients. Fifteen of these patients had invasive disease due to S. pneumoniae including 12 with meningitis, 2 with occult bacteremia and 1 with suppurative arthritis. Five of the patients with meningitis also had bacteremia. Pneumococcal antigen was detected in the cerebrospinal fluid of 9 of the 12 patients with meningitis (sensitivity 75%). However, antigen was detected in the serum of only two of the six patients with bacteremia (sensitivity 33%) and was detected in the urine of none of five patients with bacteremia (sensitivity 0%). Consequently latex particle agglutination appears to be useful when cerebrospinal fluid is examined in patients with pneumococcal meningitis but does not appear to be sufficiently sensitive to warrant its use with serum or urine in patients with invasive disease due to S. pneumoniae. The specificity of the system used here appeared satisfactory, since pneumococcal antigen was not detected in any of the body fluids from the 61 patients without evidence of invasive pneumococcal disease (specificity 100%).