It was found that the human normal immunoglobulin G product, prepared from pooled plasma of at least 1, 000 normal human donors, contained some specific IgG antibodies to Mycoplasma pneumoniae by Western blotting. M. pneumoniae-ELISA OD values increaed linearly from 25 to 800 μg/ml in the concentration of the product, there were no differences in the M. pneumoniae-ELISA OD value of the products among lots. Then we used this product as control serum for measurement of IgG antibodies to M. pneumoniae in human sera by ELISA and compared the results with those obtained by complement fixation (CF) and indirect hemadosorption (IHA) tests. ELISA titers determined by this method had a mutual relation with CF and IHA titers.