Mice given the adenosine deaminase inhibitor 2-deoxycoformycin, for five days were evaluated 24 h, 72 h and 6 days after the final dose. Spleen weight was decreased for up to 6 days after treatment. The number and relative percentage of circulating lymphocytes were decreased 24 and 72 h after the last injection. Lymphoproliferative responses to T cell mitogens were suppressed for at least 72 h post-treatment, whereas the mixed lymphocyte response was normal at 24 h but was depressed at 72 h post-treatment. Natural killer (NK) cell activity was greater in treated mice than in controls for the entire observation period. The antibody responses of mice, treated with 2 or 4 microgram 2dCF/g over four days prior to or after immunization with sheep erythrocytes, were suppressed or enhanced, respectively, compared to controls. The results indicate selective effects of 2dCF on immune function and suggest that subpopulations of lymphocytes differ in the degree of sensitivity to 2dCF.