The activation of B cell dependent T cells during Leishmania infection cannot be considered a trivial event, because their removal profoundly alters the course and outcome of infection within genetically susceptible and resistant mouse strains. The demonstration that idiotype recognizing T cells also appear within human populations sensitized to leishmanial antigens as a result of asymptomatic or subclinical infections supports a role for these cells in immunity. These cells are not demonstrable in patients with active visceral disease, so that their role in promoting specific unresponsiveness has not been extended to humans. Whether B cell dependent, idiotype specific T cells represent a functionally distinct T lymphocyte subset with unique regulatory activities remains to be determined.