The postgrafting killing of recipient’s cells is mediated by clones expressing a single specificity, for the MHC class I KIR ligand which is missing in the recipient. Shown are the alloreactive NK clones from transplants 7 and 9. KIR expression was analyzed by immunofluorescence and MHC specificity by cytotoxicity assays against cell lines transfected with the indicated class I allotypes. Results are presented as percentage inhibition compared with lysis of the untransfected cells (E:T ratio = 10:1).