Antigens secreted by M. tuberculosis in the culture medium and antigens obtained from sonicated M. tuberculosis were characterized, at the laboratory of Tropical Hygiene of the Royal Tropical Institute and the division of Pulmonary Medicine at the Academic Medical Centre, in order to explore which antigens could be valuable in the development of a serological test for tuberculosis. Using murine monoclonal antibodies in immunoblot, a pattern of protein bands distinct from that of the corresponding M. tuberculosis sonicate was found in the culture medium. The major protein bands of the culture medium, of 24 and 12 kD, and the major protein band of 16 kD in sonicate were purified. These antigens were tested in ELISA with sera from 20 patients with tuberculosis, diagnosed by a positive culture of M. tuberculosis, and from 21 control subjects. The ELISA results obtained with these 3 antigens were combined and the mean value obtained in the control group plus 2 times the standard deviation was chosen as the cut-off level. Sixteen of the 20 patients with tuberculosis had antibodies against 1 of the purified antigens, while none of the control subjects had. By combining the results, obtained with these 3 antigens, 17 of the 20 patients with tuberculosis were positive in this serological test and none of the control subjects. The 24, 12 and 16 kD antigens may be valuable for the development of a serological test for tuberculosis.