Apoptosis morphology (DNA condensation) and endonucleolytical DNA cleavage (TdT assay) were measured simultaneously on double fluorescence labelled cells employing confocal laser scanning and conventional immunofluorescence microscopy. In vitro experiments on irradiated HL-60 cells revealed a high correspondence of non-apoptotic (normal) cells without detectable DNA cleavage, versus apoptotic cells and apoptotic bodies showing DNA cleavage. Experiments performed on histological slides of testis tumours reflected a heterogeneous picture: non-apoptotic (normal) cells, apoptotic cells, and apoptotic bodies appeared either with or without detectable DNA cleavage. These data allowed the characterization and quantitation of the grade of disturbance/heterogeneity of the apoptosis programme in vivo. Furthermore, the measured apoptotic index (AI) based on apoptosis morphology was lower than the AI assessed by DNA cleavage, in contrast to published work. Taken together, these methods represent a new approach and might be suitable for improved correlation with clinical parameters. In addition, the data presented confirm frequently published doubts regarding the ability of the TdT assay to detect apoptosis as defined by morphological criteria in tumours.