One hundred patients with active tuberculosis were tested for tuberculin reactivity within 24 hours of their admission to the hospital. Commercial intermediate tuberculin, Tween stabilized intermediate tuberculin and the Tine test as well as a mumps antigen were applied simultaneously. False negative reactions were obtained in 28 per cent with Tine testing and in 21 per cent with Tween stabilized as well as plain tuberculin. These nonreactors were clinically identifiable as seriously ill with manifestations primarily attributable to protein depletion as a result of their illness. This is not specifically related to the effects of tuberculosis itself, but can be demonstrated in patients suffering the same sequelae of other debilitating illness. After two weeks of protein supplementation via a high calorie, high protein, hospital diet, skin reactivity was restored in the vast majority of these nonreactors. It is concluded that the lack of tuberculin reactivity on hospital admission probably results from impaired lymphocyte function in patients suffering serious protein depletion as a result of their illness; it is not attributable to deficiencies in the tuberculin test itself.