Enzyme ImmunoAssay (ELISA) for the determination of IgM antibodies to Chlamydia Pneumoniae in human plasma and sera. The product is intended for the follow-up of patients showing respiratory pathologies referable to Chl. 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 IgM are captured, if present, by the solid phase. After washing out all the other components of the sample, in the 2nd incubation bound anti-CP IgM are detected by the addition of anti hIgM 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-CP IgM antibodies present in the sample. The presence of IgM in the sample may therefore be determined by means of a cut-off value able to discriminate between negative and positive samples. Neutralization of IgG anti-CP, carried out directly in the well, is performed in the assay in order to block interferences due to this class of antibodies in the determination of IgM.