Cellular immune status in five patients with multi-drug resistant pulmonary tuberculosis was investigated and compared with five matched controls with non-resistant tuberculosis. A significant reduction in fresh natural killer (NK)-cell activity was found in the resistant group (P less than 0.005). There were no significant differences between the two groups in lymphocyte phenotype, proliferation or PPD-specific cytotoxicity. Reduced NK-cell function may play a role in the pathogenesis of multi-drug resistant pulmonary tuberculosis.