The diagnostic value of detection of specific IgM antibodies was analysed in Mycoplasma pneumoniae infections. In a retrospective clinical and serological study, M. pneumoniae IgM antibodies were determined by a mu-capture ELISA using enzyme-labelled antigen. The study group consisted of 91 patients with significantly raised titers in paired sera or a single high titer of complement fixation antibodies. About 40% of the patients had been treated with antibiotics ineffective against M. pneumoniae infections prior to admission to hospital. Treatment with erythromycin or tetracycline was shown to give a shorter period of fever compared to if no or ineffective therapy was given. Specific IgM antibodies were detected in about 80% of sera sampled 9 days or more after onset of symptoms. In sera sampled at 7-8 days after onset IgM antibodies were found in about 40% of the sera but only occasionally in sera sampled earlier. In the age group 0-20 years 88% of the patients developed an IgM response. In the higher ages (greater than 60 years) a significantly lower rate of IgM responders was observed.