A serum diagnostic test for tuberculosis has been devised on the basis of competitive inhibition by human sera of the binding of 125I-labelled murine monoclonal antibodies (Mabs) to a solid-phase bound pressate of M. tuberculosis. Five monoclonal antibodies binding to distinct antigenic determinants of the organism were used as structural probes which conferred their stringent combining site specificities to the polyclonal mixture of human antibodies. Sera from patients but not from healthy controls competed effectively with the binding of 125I-labelled Mabs to M. tuberculosis-coated polyvinyl plates. This inhibition technique eliminated the need for elaborate purification of antigen used in previous serological methods. Some Mabs gave considerably more positive results than others. The best combination of tests used 2 Mabs and yielded a positive result in 71% of 41 patients with smear-positive pulmonary tuberculosis. This approach is applicable in principle to the serodiagnosis of other human bacterial diseases.