CD4+ lymphocytes transfer tumour immunity to naive syngeneic mice. Donor mice had been previously treated with anti-CD25 mAbs and had rejected B16F10. Lymphocyte populations were purified from the spleens of the donor mice approximately 3 weeks after antibody administration. Recipient mice were either untreated (A, n=2) or injected either with 10⁷ purified CD4+ (B, n=3) or CD8+ (C, n=3) T cells or with 5 x 10⁷ spleen cells depleted of either CD4+ cells (E, n=3) or CD8+ cells (F, n=3). One group of mice also received 200 µl of serum recovered from the blood of the same donor mice (D, n=3). On the same day all recipient mice were inoculated s.c. with 2 x 10⁴ B16F10 cells and tumour growth was subsequently monitored. The number of mice that rejected tumour cells is indicated in each panel. Similar findings were obtained in two separate experiments.