Immunofluorescent staining has been used to identify Mycoplasma hyopneumoniae in smears of broth cultures, in infected pig testicle cell cultures, and in frozen cut sections of pneumonic lungs from field and experimentally produced cases of enzootic pneumonia. In the pneumonic pig lung, fluorescent staining was limited to the surface of the bronchial and bronchiolar epithelium and to the contained exudate. In a series of trials using experimentally infected pigs fluorescence was not detected until 25 days post-infection and was regularly seen in pigs killed thereafter. Porcine immune globulin precipitated from the serum of experimentally infected pigs and conjugated with fluorescein isothiocyanate was reactive and specific for the detection of M. hyopneumoniae. Immune globulin conjugates prepared from the serum of hyperimmunized rabbits were reactive but in some cases produced a faint non-specific staining of frozen tissue sections. No such non-specific reactions were noted on stained culture smears or cell cultures. Fluorescence was not seen in known positive preparations stained with non-immune pig globulin conjugates or in preparations from uninoculated cell cultures or pigs, stained with non-immune or immune globulin conjugates. Mycoplasma hyorhinis was detected by immunofluorescent staining with homologous conjugates, in smears of broth cultures and in tissue sections from pigs with polyserositis. Immunofluorescent staining was found to be species specific and useful for the early species identification of mycoplasma isolated from pigs.