In previous studies of the treatment of localized cutaneous leishmaniasis (LCL) we demonstrated that the therapeutic efficiency of immunotherapy (BCG plus promastigotes of Leishmania mexicana) is equal to that of chemotherapy (Glucantime), without causing the serious side-effects of the drug treatment. In the present study, various aspects of cell-mediated immunity, including the lymphoproliferative response, and leucocyte subpopulations were evaluated both before treatment and after cure in 39 LCL patients who had received immunotherapy (IT), in 34 submitted to chemotherapy (CT), and in 14 patients cured by the administration of BCG alone. We demonstrated evident signs of T-cell activation in cured patients who had received either CT or IT. For example, an increased expression of IL-2 receptors was observed in such patients, compared to their pretreatment values. Also, a significant percentage of patients showed augmented in-vitro responses to mitogen, and both in-vitro and in-vivo reactivity to leishmanial antigen. No evidence was found for the development of an exaggerated immune response to Leishmania parasites in the IT group, because the final level of immunological reactivity was comparable to the CT group. Neither was there any difference in terms of the final immune response between the patients cured by BCG treatment alone and the other groups. However, the therapeutic efficiency of BCG was lower and the mean cure time was longer. We conclude that IT is very useful in the treatment of LCL patients because of its high efficiency, low propensity to produce side-effects, and the fact that it does not induce a state of hyper-reactivity.