The purpose of the present study was to analyze Chlamydia pneumoniae-induced, antigen-specific, cell-mediated immunity. Peripheral blood mononuclear cells of four persons infected with C. pneumoniae Kajaani 6 and 17 healthy volunteers were stimulated with antigen composed of whole elementary bodies of C. pneumoniae Kajaani 6 (CP-Ag). Definitive antigen-specific lymphoproliferation (LP) responses were developed after recent infection. The LP responses of healthy people to CP-Ag varied considerably. There was no clear correlation between LP responses to CP-Ag and those to an antigen prepared from Chlamydia trachomatis serotype L2 (r > or = 0.50, P < 0.1). A larger study is required to demonstrate whether the LP responses to CP-Ag can be used for the diagnosis of C. pneumoniae infection.