The break-down of benzamide, acetamide, malonamide and allantoin in M. smegmatis was investigated. It has been stated that the uptake of liberated NH3 into the cells, favoured by the presence of an organic acid, occasionally results in a negative NH3 determination. This difficulty can be overcome by an increase of the substrate concentration from 0.8 up to 4 mM. All antoinase activity in mycobacteria can be demonstrated only by an NH3 determination, when all the enzymes necessary for the complete break-down of allantoin are present. Bacteria containing allantoinase but not urease will be negative in this test. Using high amide concentrations (4 mM) some doubtful results concerning the degradation of acetamide, benzamide, nicotinamide and pyrazinamide can be eliminated as could be demonstrated for different strains of mycobacteria.