Twenty-one patients treated for active tuberculosis were examined for immune reactivity to purified protein derivative (PPD) and to a purified 32-kDa protein antigen (P32) from Mycobacterium bovis, strain BCG. Lymphoproliferation of peripheral blood leucocytes to PPD and P32 was positive in 95% and 71% of the patients respectively. A positive IFN-gamma response was detected in 62% against PPD and in 48% against P32. Low blastogenesis and IFN-gamma production were observed, especially in patients with poor general health and advanced tuberculous lesions. Twelve out of twelve (100%) of the tuberculin-positive healthy volunteers responded to PPD and P32 with mean lymphoproliferation and IFN-gamma values that were higher than in the patient group. Twelve tuberculin-negative control subjects were completely unreactive to PPD and P32 antigen. On the other hand, IgG antibodies in the serum were detected in 95% of the patients against PPD, in 77% of the patients against P32 but in none of the tuberculin-positive or negative healthy volunteers. The highest IgG levels against PPD were found in those patients with the lowest in vitro lymphoproliferation and IFN-gamma production (r = -0.54; P less than 0.05). Nonspecific interferon production following induction with Newcastle disease virus, Corynebacterium parvum, or phytohaemagglutinin was comparable in the control and patient groups. Finally, low IFN-alpha titres were detected in the serum of about 50% of the patients.