Development of an immunological tool to detect infection with Mycobacterium leprae would greatly benefit leprosy control programmes, as demonstrated by the contribution of the tuberculin test to tuberculosis control. In a new approach to develop a 'tuberculin-like' reagent for use in leprosy, two new fractions of M. leprae depleted of cross-reactive and immunomodulatory lipids- MLSA-LAM (cytosol-derived) and MLCwA (cell wall-derived)-have been produced in a form suitable for use as skin test reagents. T cell responses (interferon-gamma (IFN-γ) and lymphoproliferation) to these two new fractions were evaluated in a leprosy-endemic area of Nepal using a simple in vitro whole blood test. The two fractions were shown to be highly potent T cell antigens in subjects exposed to M. leprae-paucibacillary leprosy patients and household contacts. Responses to the fractions decreased towards the lepromatous pole of leprosy. Endemic control subjects also showed high responses to the fractions, indicating high exposure to M. leprae, or cross-reactive mycobacterial antigens, in this Nepali population. The new fractions, depleted of lipids and lipoarabinomannan (LAM) gave enhanced responses compared with a standard M. leprae sonicate. The cell wall fraction appeared a more potent antigen than the cytosol fraction, which may be due to the predominance of the 65-kD GroEL antigen in the cell wall. The whole blood assay proved a robust field tool and a useful way of evaluating such reagents prior to clinical trials.