The indirect fluorescent antibody technique for specific IgM and IgG antibodies was applied to paired sera from 33 patients with a current M. pneumoniae infection, along with the complement fixation test. A roughly parallel increase in antibody titres in all the three tests was observed; the ratio of IgM/IgG titres was, however, higher in patients below 20 years of age than in older patients. Rises in antibody titres were regularly observed in spite of the fact that most of the patients were treated with tetracycline or erythromycin during the acute phase of the disease. Serum specimens from 15 other patients lacking clinical data of a recent M. pneumoniae infection, but with stationary complement fixation titres, all showed IgG antibodies with stationary titers and, with the exception of three patients, a negative IgM antibody test. Sera from 20 individuals lacking complement fixing antibodies were also without demonstrable IgG and IgM antibodies. Sampling of serum from 9 of the pneumonia patients was repeated. The last IgM positive sample was collected up to six months after onset of the disease and all were found to be negative in the IgM test 8 to 10 months after onset of the illness in these treated patients. The implications of these findings for the serological diagnosis of M. pneumoniae infections are discussed.