The antibody response to individual epitopes has previously been analysed by competition assay using 125I- or enzyme labelled monoclonal antibodies. A modification of the test is described in which competition of human sera with unlabelled mouse monoclonal antibodies at the limiting dilution is revealed by peroxidase labelled antimouse IgG conjugate. Analysis of 54 sera from patients with pulmonary (36) and extra-pulmonary (18) tuberculosis and 31 controls indicated that the modified test compares favourably with the test based upon directly labelled antibodies. Diagnostic sensitivity for five monoclonal antibodies evaluated was 11.1% (TB78), 35.2% (TB23 and TB68), 37.0% (TB71) and 61.1% (TB72) at 97.5% specificity. For TB72, sensitivity was highest for pulmonary disease (69.7%). The modified assay is also easier to standardise for screening new monoclonal antibodies using a single enzyme-labelled conjugate.