5'-Nucleotidase activity was first described in heart and skeletal muscle about 60 years ago (Reis, 1934). It catalyses the hydrolysis of phosphate esterified at carbon 5'of the ribose and deoxyribose portions of nucleotide molecules (EC 3.1. 3.5). Activity of 5'-nucleotidase has been described for bacteria and plant cells and the enzyme is also widely distributed in vertebrate tissues. Enzymes hydrolysing 5'-nucleotides from the various sources display significant differences in the range of substrates hydrolysed as well as in substrate specificity. Furthermore, there occur different cellular locations. 5'-Nucleotidase activity is found not only in soluble but also in membrane-bound, surface-located form. In addition to a broad spectrum of 5'-purine and pyrimidine mononucleotides, 5'-dinucleotides and 5'-trinucleotides, or even complex nucleotides like UDP-glucose or FAD, can also be hydrolysed by the various 5'-nucleotidases. The abilityto hydrolyse 5'-mononucleotides appears to be the common denomi-nator. This raises thequestion as to whether the 5'-nucleotidases at different phylogenetic levels or cellular locations belong to one principal type of enzyme or if they represent different forms of enzymes (and proteins) with partially overlapping ranges of substrates. Recent analyses of the primary structures derived from cDNAs of bacteria and vertebrates have provided a means to group at least part ofthe enzymes so far described into one family of phylogenetically related proteins. Whereas cytosolic 5'-nucleotidase activity controls intracellular levels of nucleoside 5'-monophosphates, surface-located 5'-nucleotidase is a major contributor to the cascade that completely hydrolyses extracellular ATP to adenosine, and thus of major pharmacological interest. The physiological function of the enzyme probably differs in various organisms and tissues, and possibly extends beyond its catalytic activity. Thus for example surface-located 5'-nucleotidase anchored to the plasma mem-brane by glycosyl phosphatidylinositol has been implicated in cell-matrix or cell-cell interactions, and even intransmembrane signalling. 5'-Nucleotidase can be transiently expressed in certain cell types during development and the activity of 5'-nucleotidase in cultured cells can be regulated by external factors. It may even be released into the extracellular medium. Various aspects of 5'-nucleotidase molecular properties and function have been reviewed (Bodansky & Schwartz, 1968; Drummond & Yamamoto, 1971; Arch & Newsholme, 1978; Fox, 1978; Riordan & Forstner, 1978; Stone, 1981; Pearson, 1985, 1987; Dieckhoff et al., 1986a; Dornand et al., 1986; Gutensohn & Rieger, 1986; Karnovsky, 1986; Kreutzberg et al., 1986; Luzio et al., 1986, 1987; Newby & Worku, 1986; Slakey et al., 1986; Widnell et al., 1986; Grondal & Zimmermann, 1988). Alterations of 5'-nucleotidase levels in a considerable number of diseases have been recognized (Sunderman, 1990). The present review focuses on recent developments in the analysis of the molecular structure and of functional aspects of 5'-mononucleotide-hydrolysing enzymes. It suggests a classification and divides the vertebrate 5'-nucleotidases into four distinct groups according to biochemical properties and cellular location.