In vitro studies indicate that in lymphomas, execution of apoptosis involves activation of effector caspases. To investigate activation of effector caspases in vivo in biopsy specimens of lymphomas, a new assay was developed using antibodies against active caspase 3 and p89, a protein fragment generated by caspase-specific cleavage of poly-ADP ribose polymerase (PARP). Using this assay, it was found that in B-cell lymphomas, levels of active caspase 3/p89-positive cells correlate strongly with morphologically recognizable apoptotic cells. The number of active caspase 3/p89-positive cells was low in follicular lymphomas and usually high in diffuse large cell lymphomas. Highest numbers were found in Burkitt lymphomas and in two biopsies of diffuse large B-cell lymphomas (DLCLs) obtained several days after initiation of therapy. It is concluded that apoptosis in reactive lymphoid tissues and in B-cell lymphomas always involves activation of effector caspase 3 and cleavage of one of the major effector caspase substrates, PARP-1. Moreover, levels of effector caspase activation are constantly low in low-grade follicular lymphomas and vary considerably in DLCL and Burkitt lymphoma.