To test if freshly isolated tumour‐infiltrating lymphocytes (TIL) can induce apoptosis in a target cell, we have combined two previously described methods. Because TIL predominantly are T‐lymphocytes. we have applied a redirected approach. When the target cells that express anti‐human‐CD3 monoclonal antibodies in their membranes bind to the T cell receptor‐associated CD3‐complex, signals are generated, which activate T cell effector mechanisms. This approach circumvents problems with MHC‐restriction and allows for functional testing of all T cells, irrespective of their clonal specificity.In order to assay for induction of DNA fragmentation, we have labelled the target cell nuclei with [3H]thymidine. Upon harvesting fragmented DNA are washed away. Electrophoretic analysis of the fragmented DNA demonstrated the characteristic ‘ladder’ pattern, consistent with apoptosis.This rapid and simple assay monitors the capacity of different T cells to induce apoptosis in the target cell. It depends on intercellular interactions and clearly discriminates between different T cell subsets. With this assay we demonstrate the functional integrity of the cytotoxic effector arm of fre