Pneumococcal surface adhesin A (PsaA) is a 37-kDa common protein antigen of Streptococcus pneumoniae. In the present study, the protein was purified so that its immunoreactivity could be determined. PsaA was released and purified from cells by lysis in the presence of n-laurylsarcosine; this was followed by ammonium sulfate precipitation and subsequent preparative isoelectric focusing. A capture antibody enzyme-linked immunosorbent assay was used to determine the immunoreactivity of purified PsaA. The assay had a 67% sensitivity for sera from patients with bacteremic pneumococcal pneumonia. A specificity of 97% was estimated on the basis of a lack of reactivity with sera from patients with pneumonia caused by other organisms. PsaA is a potential vaccine candidate and may be useful as an antigen in a diagnostic assay for pneumococcal disease.