Clinical, morphological and immunological studies of human tuberculosis have enabled the spectrum of the disease to be determined. We have investigated the cell-mediated immune responses by means of skin tests and leucocyte migration inhibition to PPD, and the humoral immune responses by means of immunodiffusion and haemagglutination tests. Patients with tuberculosis can be classified into two polar groups--reactive (RR) and unreactive (UU), the former showing good cell-mediated immunity and little or no antibody formation and the latter poor cellular responses and exuberant antibody production. The intermediate forms show characteristics of the neighbouring polar groups. The existence of a spectrum of immune response in tuberculosis, which has long been suspected, is now demonstrated.