The possible benefits for improved diagnosis of extrapulmonary tuberculosis were evaluated by means of a modified serological competition assay. Antibodies to the 38 kD Mycobacterium tuberculosis antigen were detected in 73% of cases of extrapulmonary tuberculosis and 70% of smear-negative cases of pulmonary tuberculosis at a chosen specificity of 98%. Of 55 patients (45%) whose diagnosis of tuberculosis was not made until 7 days or longer after admission, 76% were positive for the antibody. In this group the serological test would have allowed chemotherapy to be started on average 12.5 days earlier and would have obviated the need for 80% of diagnostic biopsies.