The mu-capture, the ELISA, IF and CF tests were used to study Mycoplasma pneumoniae infections in Cambridge in 1983. We studied 393 samples of serum submitted for investigation of M. pneumoniae infection, from 298 patients. There was a good correlation between mu-capture ELISA and IF for detecting M. pneumoniae-specific IgM, and between elevated CF titer, and the presence of M. pneumoniae-specific IgM. There was also a clear correlation between M. pneumoniae-specific IgM status and age. M. pneumoniae-specific IgM and IgA were most often found together in sera, but in some sera, only one of these antibodies was found at significant levels. These findings were not age related, and thus not obviously useful in differentiating primary and recurrent M. pneumoniae infections.