A method is described for the identification of virulent mycobacteria. The mycobacteria are cultivated on filter paper or molecular filter membrane matrices, then the microcolonies are treated in situ with neutral red and alkali, and observed under the microscope. Stock virulent tubercle bacilli cultures and all strains of tubercle bacilli recovered from resected pulmonary tissues were positive by both criteria; but the saprophytes as well as the chromogens and other mycobacteria of questionable pathogenicity were either neutral red negative, non-corded, or negative in both respects.