T cells from alloimmunized and unimmunized donors that proliferate in response to K peptides have previously been activated in vivo. Proliferative responses of CD45RO⁺ (previously activated/memory) and CD45RA⁺ (previously inactive/naive) T-cell fractions from alloimmunized donors 1 and 5 and unimmunized control donors 4 and 5 against selected Kell peptides. The results are representative of 4 alloimmunized and 4 control donors.