A comparison is being made of the immune response to PPD (purified protein derivative of tuberculin .6 to 6.5 mg per milliliter protein) and MT (mammalian tuberculin - USDA reference strength) using natural cases of primate tuberculosis. Results indicate that the hypersensitivity response to MT is linear, increasing with the amount of protein present. The hypersensitivity response to PPD is not linear, plateauing at approximately 2.0 mg per milliliter. The response was consistently of greater intensity to MT than to PPD, when equivalent strengths above .6 mg per milliliter of protein were used. MT appears to be the better product for diagnosing tuberculosis in rhesus monkeys.