Natural killer (NK) cells can lyse a variety of different tumour cells by exocytosis of perforin, subsequent binding of perforin to the target cell membrane and formation of lytic pores. Some tumour cells, however, are resistant to cellular cytotoxicity. Using the NK-resistant tumour cell lines ML-2, MONOMAC-1, RPMI and L540Cy, we demonstrated that activation of NK cells with interleukin 2 (IL-2) and IL-12 resulted in significant lysis of these tumour targets. To investigate the underlying mechanisms, we isolated the cytotoxic granules from non-activated and IL-2-/IL-12-activated NK cells and compared the killing of K562 leukaemia cells (sensitive to NK cell-mediated lysis) and ML-2 leukaemia cells (resistant to NK cell-mediated lysis). In contrast to K562 cells, which were easily killed by NK-cell granules, ML-2 cells were resistant to granules from non-activated NK cells. However, granules from NK cells activated with IL-2 and IL-12 were able to induce significant tumour cell lysis. Cell death of both K562 and ML-2 cells by granules from activated NK cells was completely blocked by anti-perforin antibodies, indicating that perforin mainly accounts for the lysis induced by NK granules. Comparing granules from non-activated and IL-2-/IL-12-activated NK cells, the increased cell death of ML-2 cells was caused by an improved binding of perforin to the target cell membrane. Functional assays, however, indicated that the differences in perforin binding were not as a result of an augmented production of perforin by activated NK cells. We conclude that activation of NK cells results in an increased binding of perforin and subsequent lysis of tumour cells.