The PNB medium is an egg medium (Ogawa egg medium or Lowenstein-Jensen medium) containing 0.5 mg of p-nitrobenzoic acid per ml. To prepare it, pnitrobenzoic acid is dissolved in propylene glycol at a concentration of 25 mg/ml and one volume of this solution is added to 50 volumes of egg medium before sterilization. The medium serves for differentiating tubercle bacilli from other mycobacteria. Mycobacter ium tuberculosis, M. bovis, M. africanum and M. microti do not grow on this medium after incubation at 37°C for 3 weeks, while all other mycobacteria do grow. The medium is useful for differentiating non-tuberculous mycobacteria, including niacinpositive M. simiae strains and niacin positive variants of other species, from the M. tuberculosis complex and for differentiating the M. tuberculosis complex strains, including niacin negative variants, from other mycobacteria (Table 5).