Guinea pigs are quite susceptible to infection with either Mycobacterium tuberculosis or M bovis. Lesions are most often seen in the parenchyma of the lung and adjacent lymph nodes. Also, guinea pigs are susceptible to certain serovars of M avium complex with lesions seen in lymph nodes associated with the GI tract. Tuberculin skin tests can be conducted by injection of purified protein derivative of M bovis and of M avium (1:100 dilution containing 5,000 tuberculin units) at separate sites in the skin of the abdomen. The preferred injection site is 2 cm posterior to the xyphoid cartilage and 2 cm lateral on each side of the linea alba. The injection sites should be observed at 24 and 48 hr for induration and swelling. The presence of erythema at the injection site is of little or no significance.