Apoptosis (programmed cell death) is a basic physiological process which determines specific patterns of tissue size and shape, and balance of cell number, during morphogenesis, and seems to play an integral role in oncogenic progression. Since dramatic changes of cellular glycosylation pattern are well known to be closely correlated with differentiation, development and oncogenesis, it is likely that similar specific changes are associated with apoptosis. However, this possibility has not been systematically investigated. We therefore carried out histological studies of many tumours and normal tissues for which a high incidence of apoptosis is believed to occur. Sections were stained with monoclonal antibodies (MoAbs) directed to carbohydrate antigens Le^y and Le^x, proliferating cellular nuclear antigen (PCNA) and Fas (previously claimed to be an apoptosis-inducing antigen). Antibody staining patterns were compared with morphological cell characteristics as revealed by haematoxylin/eosin staining, and DNA fragmentation patterns (a marker of apoptosis) as revealed by 3'-OH nick-end labelling technique. We found that expression of Le^y (defined by MoAb BM1) is closely correlated with the process of apoptosis, but not with cell proliferation or necrosis. Within Le^y-positive areas of tissue sections, typical apoptotic morphological changes and DNA fragmentation (as revealed by positive nick-end labelling) were frequently observed in certain loci, although not all Le^y-positive cells showed such signs of apoptosis. Le^y-positive areas showed consistent negative staining by MoAb directed to PCNA and negative or weak staining by MoAb directed to Fas antigen, regardless of tissue source. No such trends were observed for Le^x glycosylation. We conclude that Le^y expression is a useful phenotypic marker predictive of apoptosis, i.e. some (although not all) Le^y-positive cells subsequently become apoptotic.