The fluorescent-antibody method has been employed for the rapid identification of Mycoplasma colonies growing on agar plates. The method was found to be effective for detection of mixtures of Mycoplasma serotypes growing on primary isolation plates. The technique also helped to define the presence of mycoplasmas which did not produce typical colonies. It was also possible to identify Mycoplasma colonies overgrown by bacterial or fungal contaminants. Conjugates directed against 10 distinct Mycoplasma serotypes have been successfully employed in this system. One of the serotypes is a human oral isolate which has not been previously characterized. The definitive identification of mycoplasmas requires antigenic analysis. A number of techniques have been employed for this purpose. Complement fixation, agglutination, indirect hemagglutination, metabolic inhibition, and growth inhibition have all been used to define the antigenic relationships of mycoplasmas, but these techniques do not allow prompt serotyping of new isolates. The common occurrence of mixed cultures of different serotypes makes it necessary to clone all cultures and use a number of single colonies for the preparation of large stocks of antigen. Employing the standard procedure ofcloning three to five colonies, there is little assurance that all strains in a mixed culture will be detected. These