The impact of human leucocyte antigen (HLA) class I expression intensity and killer-cell immunoglobulin-like receptor (KIR)-mismatch was investigated on natural killer (NK)-cell mediated lysis of B-lineage leukaemic blasts from 21 paediatric patients in vitro. Blast susceptibility to standardised NK-activity and HLA-expression differed widely. A clear association between HLA-molecules/cell (range 442 000-13 000) and specific lysis (mean 59%, range 13-98%) was observed (r(2) = 0.68). A compound model incorporating HLA-expression and KIR-ligand-mismatch provided an even stronger association (r(2) = 0.87), whereas KIR-ligand-mismatch alone was not significant. Assessment of these factors might identify patients who could benefit from NK-mediated graft-versus-leukaemia effects after mismatched stem cell transplantation.