A sensitive indirect ELISA is reported for the detection and quantitation of specific IgG to Mycoplasma gallisepticum (MG) in sera and tracheobronchial washes (TBW) of MG-infected chickens. The sensitivity of the assay was ensured by the use of mouse monoclonal antibody to chicken IgG bound to a prospective anti-MG containing sample that was complexed with MG antigen immobilized on a solid phase. The level of specific IgG antibody in a test sample was detected by using peroxidase-conjugated goat anti-mouse IgG. Serum samples with various levels of anti-MG IgG activity were used to construct a standard curve response at a single working dilution by using Logit-Log curve fitting. The assay was simple, reliable, and specific and was used to monitor the appearance of specific anti-MG IgG in chicken sera and TBW at various intervals after the onset of mycoplasma-induced respiratory disease. The IgG response reached a plateau at 2 and 4 weeks postinfection in TBW and sera, respectively; then the response waned but still was detectable at a significant level for up to 25 weeks postinfection.