Summary A selective medium for Mycobacterium tuberculosis and Myco. bovis has been developed containing polymyxin B 200 units per ml, carbenicillin 100 γ per ml, amphotericin B 10 γ per ml and trimethoprim 10 or 20 γ per ml in 7H10 oleic acid albumin agar. When the centrifuged deposit from sputum, liquefied by treatment with dithiothreitol, was plated directly on to the selective medium, less than 1 per cent. of plates were contaminated and the viable counts were about 240 per cent. higher than parallel counts after treatment of the deposit with NaOH and plating on 7H10 medium without drugs. A comparison of results on medium with and without the selective drugs showed that the drugs slowed the early growth of Myco. tuberculosis in slide culture and reduced viable counts by, at most, 10 per cent. Improved anti-fungal cover could be obtained with no reduction in viable counts by increasing the concentration of amphotericin B to 50 γ per ml.