Forty-six of sixty-eight rural Nigerian adult patients with sputum-positive pulmonary tuberculosis (P.T.B.) had skin reactions of less than 10 mm. in response to 4 or 5 tuberculin units of purified protein derivative (P.P.D.). The size of the reaction correlated significantly with serum albumin and transferrin concentrations. The diminished reaction in these patients seems to be part of a generalized depression of cell-mediated immunity associated with undernutrition rather than a specific depression of P.P.D. reactivity associated with overwhelming antigen load. It is concluded that the tuberculin test is of no value in the diagnosis of P.T.B. in this community, either in individual cases or epidemiological surveys, unless combined with a biochemical assessment of the person's nutritional status.