Purine nucleoside phosphorylase (EC 2.4.2.1) activity in human erythrocytes was measured using capillary electrophoresis. Enzyme samples were directly loaded into the capillary without preconcentration or purification. The electrophoretic separations were carried out in an uncoated fused-silica capillary (75 microm internal diameter, effective length of 45 cm, total 72 cm) at an electric field of 415 V/cm at ambient temperature (25 degrees C). UV detection at 200 nm was used. Borate buffer (100 mmol/l, pH 9.5) was used as background electrolyte. The results obtained by CE compared favourably (r=0.989) with those of standard HPLC methods. The method presented is reliable, slightly faster and less expensive than the routinely performed HPLC method.