Apoptosis, or programmed cell death, is an endogenous cellular process whereby an external signal activates a metabolic pathway that results in cell death. This form of cell death appears to be a common feature in many biological processes where cell deletion is a mechanism for altering tissue structure and function. Historically, apoptosis has been studied using histological techniques; however, more recent interest has focused on analyzing this process at the biochemical level. A biochemical hallmark of apoptosis is a characteristic form of DNA degradation in which the genome is cleaved at internucleosomal sites, generating a 'ladder' of DNA fragments when analyzed by agarose gel electrophoresis. A number of assay systems have been developed to study this nuclease activity. For example, nuclease activity has been analyzed by measuring the release of endogenous DNA from apoptotic cells, by flow cytometric analysis of apoptotic cells and by analyzing in situ apoptotic nuclease activity in polyacrylamide gels containing DNA. Use of these assay systems has enabled investigators to study the signal transduction pathways that mediate apoptosis and to characterize the endonuclease itself. Future biochemical studies in this field will focus on isolating the genes and gene products that mediate apoptosis.