A brief description of current procedures for the Chlamydia microimmunofluorescence (MIF) test is presented. To date, use of MIF serology with Chlamydia pneumoniae (TWAR) antigen has provided the most sensitive and specific method for diagnosis of acute TWAR infection. In primary infections, the TWAR IgM antibody response is longer lasting and IgG antibody is slower to develop compared with Chlamydia trachomatis infection. Unlike other Chlamydia species, only a single serovar for C. pneumoniae has been recognized in the MIF system and cross-reaction with other species is negligible. While IgM antibody response is an important marker for serodiagnosis of acute infection, rheumatoid factor often causes false-positive reactions. Persistent TWAR IgG antibody has been useful for seroepidemiologic studies and an association of TWAR IgG antibody and atherosclerotic diseases has been observed. IgA antibody may not be a useful marker for chronic TWAR infection or for acute infection.