METHODS Many authors have shown rising titres of anti-mycobacterial antibodies after a few months of anti-tuberculosis treatment. This humoral response might persist for years, making the discrimination between current and old disease difficult. OBJECTIVE Characterisation of the humoral response to culture filtrates of Mycobacterium tuberculosis before and after treatment of tuberculous patients in order to identify those antigens that could provide information about disease activity. METHODS Anti-mycobacterial IgG response was determined during and after treatment of tuberculous patients by ELISA and immunoblot. Serum was taken from 71 active tuberculous patients (59 newly acquired and 12 relapse), 15 old tuberculous patients and 45 nontuberculous control subjects. RESULTS By ELISA, antibody response increased after 2 months of treatment. After chemotherapy was completed, the estimated number of antibodies remained at the same level. The level of specific antibodies in patients seems to reach the same level as that of control subjects 3 years after initiation of treatment. In Western blot, although each patient serum had its own characteristic banding pattern, differences between tuberculous patients and control subjects were found in the area below 20 kDa. Serum from tuberculous patients showed high levels of antibodies at the 14 kDa region. After the beginning of treatment, the intensity of the 14 kDa region band and the percentage of positive recognition tended to decrease. Therefore, one year after initiation of treatment, only seven of 13 cases who demonstrated anti-mycobacterial antibodies in ELISA revealed a mild but still positive reaction at the 14 kDa region; this reactivity disappeared 2 years after initiation of chemotherapy. CONCLUSIONS The 14 kDa region antigen seems to induce a humoral response that evolves in relation with the disease activity.