Enzyme ImmunoAssay (ELISA) for the quantitative determination of IgG antibodies to Chlamydia pneumoniae in human plasma and sera. The kit is intended for the follow up of patients undergoing a Chlamydia pneumoniae infection. Micro-plates are coated with a preparation of native C.pneumoniae. In the 1st incubation, the solid phase is treated with diluted samples and anti-C.pneumoniae IgG are captured, if present, by the solid phase.After washing out all the other components of the sample, in the 2nd incubation bound anti-C.pneumoniae IgG are detected by the addition of anti hIgG antibody, labeled with peroxidase (HRP). The enzyme captured on the solid phase, acting on the substrate/chromogen mixture, generates an optical signal that is proportional to the amount of anti-C.pneumoniae IgG antibodies present in the sample. IgG in the sample may be quantitated by means of a standard curve calibrated in arbitrary units per milliliter (Uarb/ml) as no international standard is available.