Antibody activity against Mycobacterium tuberculosis of sera from an area with a high prevalence of tuberculosis was measured by enzyme-linked immunosorbent assay (ELISA) with a plasma-membrane extract from M. tuberculosis strain H37RV. All sera from relapsed tuberculosis patients and 82.5% of sera from new untreated cases gave positive results. The seronegative group of tuberculosis patients gave positive results by direct microscopy and culture. No clear correlation between antibody and delayed hypersensitivity or extent of disease was observed. Chemotherapy was associated with a higher antibody response. Specificity of the test with healthy control subjects from the high prevalence area was 85%. Negative results were obtained with 145 sera from presumed healthy European subjects and with seven sera from BCG-vaccinated subjects.