A purine 5'-nucleotidase has been separated by DEAE-Trisacryl chromatography from other 5'-nucleotidase activities present in human haemolysates and purified approx. 30,000-fold by subsequent chromatography on Blue Sepharose. The enzyme has an Mr of around 250,000, displays hyperbolic substrate-saturation kinetics and hydrolyses preferentially IMP, GMP and their deoxy counterparts. It is much less active with AMP and dAMP. The purine 5'-nucleotidase is inhibited by Pi, and is strongly stimulated by ATP, dATP and GTP, and by glycerate 2,3-bisphosphate. Stimulators decrease Km and increase Vmax. Glycerate 2,3-bisphosphate is the most potent stimulator of the enzyme and, under physiological conditions, over-rides the influence of the other effectors. Glycerate 2,3-bisphosphate also influences the binding of the enzyme to DEAE-Trisacryl, as evidenced by the different elution profile obtained with fresh as compared with outdated blood. It is concluded that the glycerate 2,3-bisphosphate-stimulated purine 5'-nucleotidase is responsible for the dephosphorylation of IMP and GMP, but not of AMP, in human erythrocytes.