BACKGROUND: The in vivo effects of immunosuppressants on T cells are classically determined using animal models of organ transplantation. These methods are technically difficult and time consuming. A simple in vivo method is needed for screening new immunosuppressants. METHODS: Donor mouse spleen cells were labeled with a fluorescent dye, carboxy-fluorescein diacetate succinimidyl ester (CFSE), and then injected into the blood of recipient severe combined immunodeficiency mice. Three days after the injection, spleen cells of the recipient mice were isolated and the proliferating alloreactive T cells were analyzed by flow cytometry. RESULTS: In control recipient mice, 50% of the T cells were proliferating, consisting of both CD4+ and CD8+ T cells. In cyclosporine- or FK506-treated mice, T-cell proliferation was suppressed in the CD4 subset but not in the CD8 subset. On the contrary, T-cell proliferation was significantly reduced in the CD8 subset but not in the CD4 subset in recipient mice treated with rapamycin. CONCLUSION: The present mouse model using carboxy-fluorescein diacetate succinimidyl ester labeling is simple and fast. It is useful for screening new immunosuppressants and for examining the effect on T-cell subsets.