The volunteers studied included 6 tuberculoid and 11 lepromatous leprosy patients (Ridley and Jopling). Each received intradermally 0.1 ml of lepromin (Wade method) and 0.1 ml of autoclaved BCG (grown on Dubos medium) at 2 separate sites on the left upper arm. Observations were made at weekly intervals, and biopsy specimens were taken at the fourth wk. Within 3 to 4 wk all 6 tuberculoid patients responded to lepromin with typical granulomas (5 of which measured 5 mm or more in diameter) composed of epithelioid cells, lymphocytes and giant cells. In contrast, 8 of the 11 lepromatous patients remained negative; the other 3 developed tiny papules less than 3 mm in diameter which disappeared after 2 wk. Biopsy specimens taken from injection sities at the fourth wk showed essentially normal histology. All 1 patients developed typical granulomas to heat-killed BCG (non were less than 5 mm in diameter and most were larger) consisting of epithelioid cells, many lymphocytes and a few giant cells. The reaction in tuberculoid leprosy was stronger than that in lepromatous leprosy and the reaction to BCG in tuberculoid leprosy was stronger than that to lepromin, but the differences were not very remarkable.