In mycoplasmal pneumonia, both bacteriological and serological diagnoses are retrospective and, therefore, offer little help to clinicians. The isolation and identification of M. pneumoniae (Mp) require more than 1-2 weeks, and the serum antibody titer takes 1-2 weeks to rise after onset of the disease.I have developed a rapid test which applies the indirect immunofluorescent technique, for patient throat smears.Anti-Mp rabbit IgG was prepared by immunizing a rabbit with a concentrated cell suspension of Mp. Purified anti-Mp rabbit igG (primary antibody) and FITC-labeled anti-rabbit IgG goat IgG were applied to the throat smears which were pretreated with normal goat IgG to eliminate background non-specific fluorescence.Specific immunofluorescence was seen in 9 out of 10 throat smears from patients with Mp infection. Both granular and diffuse types of fluorescence were seen in the smears; the former was found in mucus and the latter on the entire surface of the epithelium. No specific immunofluorescence was seen in smear taken from patients with other respiratory infections or from normal volunteers.I consider that the method is useful for the early and rapid diagnosis of mycoplasmal infection.