Administration of recombinant interleukin 2 (rIL 2) resulted in a significant (p less than 0.01) decrease in mortality in mice infected with a dose of Toxoplasma gondii that killed 100% of untreated mice. Mice treated with rIL 2 had a significantly (less than 0.005) lower numbers of cysts in the brains. The protection afforded by rIL 2 could not be correlated with increased antibody synthesis or be explained by increased macrophage killing in the treated mice. Mice treated with rIL 2 after Toxoplasma infection demonstrated increased natural killer (NK) cell activity compared with either Toxoplasma-infected or rIL 2-treated mice. rIL 2 failed to reverse the suppressed proliferative response of lymphocytes to concanavalin A and lipopolysaccharide in mice acutely infected with a virulent strain of T. gondii. These results reveal that rIL 2 may have a remarkably protective effect against intracellular parasites.