The enzyme linked immunosorbent assay (ELISA) was used for estimation of the IgG and IgM antibody responses against 13 type specific pneumococcal capsular polysaccharides, individually and in a pool, in 52 patients with pneumococcal pneumonia and in 20 control patients with non-pneumococcal pneumonia or enterobacterial septicemia. By use of the isolated type 3 polysaccharide as antigen a greater than or equal to 50% increase in absorbance from acute to convalescence phase serum, equivalent to a doubling in antibody titer, was seen significantly more often in 22 patients with type 3 pneumococcal pneumonia than in the 20 control patients (for IgG 14 vs. 3, p less than 0.01; for IgM 14 vs. 4, p less than 0.01). However, for an acceptable degree of specificity to be obtained (less than or equal to 10% control patients positive) a doubling of the IgG or IgM absorbance values had to be demanded. With this criterium only half of the patients infected with pneumococcal types included in the antigen set up used, showed a type specific antibody response and only one third of all patients with pneumococcal pneumonia could be diagnosed by use of the 13 polysaccharides as a pool antigen.