In order to study the role of natural killer (NK) cells during the early period of Leishmania infection, BALB/c mice were selectively and permanently depleted of NK cells by injection with 90 Sr and subsequently infected with Leishmania (Leishmania) amazonensis (HSJD-1 strain). 90 Sr is known to selectively deplete NK cells, leaving an intact T-and B-cell compartment and preserving the ability to produce both interferon alpha and IL-2. This method of depletion has advantages when compared with depletion using anti-NK cell monoclonal antibodies because the effect is permanent and neither activates complement nor provokes massive cell death. In the present study, after one month of treatment with 90