Cytosolic 5 -nucleotidase, acting preferentially on IMP, GMP and their deoxyderivatives, can also behave as a phosphotranferase, operating a transfer of phosphate from a nucleoside monophosphate donor to a nucleoside acceptor which, besides a natural nucleoside, can be also an analog. The enzyme activity is stimulated by ADP, ATP and 2,3-bisphosphoglycerate (BPG). The concentration of effector required to attain half maximal activation (A 0 . 5 ) for the bisphosphorylated compound is in the millimolar range, so that BPG seems to act as a physiological activator of 5 -nucleotidase only in erythrocytes. However, the combination of BPG and ADP brings about a significant increase of their respective affinity for the enzyme, lowering their A 0 . 5 values approx. 4-times. The observation that BPG favors the phosphotransferase more than the hydrolase activity of 5 -nucleotidase stands for a key role of this metabolite in the regulation of the processes of activation of purine pro-drugs, in which this enzyme is involved.