Populations of peripheral blood T lymphocytes from patients with Kenyan visceral leishmaniasis were studied using specifically defined antisera (monoclonal antibodies, Ortho-mune® OKT3, OKT4, OKT6, and OKT8). The levels of total T lymphocytes and circulating thymocytes were within the same range as those of clinically normal individuals. However, the proportions of the helper/inducer T cells were lower in untreated patients than in the controls (18.9% vs. 39.7%) while the levels of suppressor/cytotoxic T cells were higher than in the controls (40.5% vs. 27.8%). After successful antileishmania treatment these levels showed a gradual return towards normal over a period of one year. It was concluded that immunosuppression observed is due to the levels of peripheral blood helper/inducer and suppressor/cytotoxic T lymphocytes.