We examined apoptosis in 33 gastric carcinomas using the terminal deoxynucleotydil transferase mediated dUTP-digoxigenin nick end labelling technique (TUNEL). Of the tumours, nine were well-differentiated, 13 moderately differentiated and 11 poorly differentiated. In addition, we also analysed MIB-1, a cell proliferation antigen. Morphologically, apoptotic tumour cells were more frequently observed in well-differentiated tumours. In addition, apoptotic signals of the TUNEL method were seen even in the nuclei of tumour cells which did not show apoptosis. The nick end labelling index was 51.0 +/- 26.3 in the well-differentiated and moderately differentiated tumours and 28.0 +/- 18.8 in poorly differentiated tumours. The mean of apoptotic body index and nick end labelling index were both significantly higher in well-differentiated and moderately differentiated tumours than in the poorly differentiated type (P < 0.0001, P = 0.008). The MIB-1 labelling index and higher in poorly differentiated tumours than in the well-differentiated or moderately differentiated tumours, and labelled cells were more numerous in the superficial region than in the middle and deep regions of tumours. No apparent correlation was found between the nick end labelling index and the MIB-1 labelling index. The high number of apoptotic cells (the high Nick end labelling index) and low proliferation potentiality (the low MIB-1 labelling index) in well-differentiated gastric carcinomas may thus be related to their natural tendency to demonstrate slow growth.