The effects of repeated injections of transfer factor over a period of 20 weeks were investigated in fourteen bacteriologically positive patients at the lepromatous side of the leprosy spectrum. All patients showed negative (0 mm induration) skin tests to M. leprae antigens (i.e. leprolin and lepromin). Of these patients, seven were treated with transfer factor with a total of 9 units (1 unit being equivalent to 5 x 10(8) lymphocytes) and seven with a placebo. Maintenance treatment with clofazimine was continued. Transfer factor was prepared from the lymphocytes of donors who showed positive skin tests to M. leprae antigens (i.e. leprolin greater than or equal to 12 mm induration, average 15.5 mm or lepromin greater than or equal to 8 mm induration, average 13.6 mm), as well as a positive lymphocyte transformation in vitro to M. leprae (the average transformation being higher than the average transformation of lymphocytes of tuberculoid leprosy patients). No differences were found between the two groups as regards the clinical course of the disease, the histopathological and bacteriological evaluation of skin biopsies, changes in skin test reactivity to various antigens (i.e. lepromin, leprolin, PPD, Mumps, C. albicans, Tr. rubrum and Varidase), as well as the lymphocyte transformation in vitro to various mitogens (i.e. PHA, PWM, Con A) and antigens (i.e. M. leprae, leprolin, PPD, BCG, Mumps, C. albicans, Trichophyton and Varidase). No evidence was found to suggest that transfer factor is a valuable adjuvant in the treatment of lepromatous leprosy patients or that it increases cell-mediated immune reactivity towards M. leprae.