Posttransplant NK cell clones of donor origin lyse the allogeneic recipient’s target cells. Donor NK cell clones obtained from the circulation of the engrafted recipients were tested for lysis of the allogeneic recipient’s target cells (PHA lymphoblasts or BLCLs obtained from cryopreserved pretransplant recipient’s lymphocytes) (▪), and of the autologous donor’s cells (□). E:T ratio = 10:1. Shown are cytotoxicity assays with NK cell clones from four representative recipients (see Table 3 for a summary of data).