Pyrimidine 5′-nucleotidase (5′-ribonucleotide phosphohydrolase, E.C. 126.96.36.199) (P5N) deficiency is one of the more common causes of hereditary non-spherocytic haemolytic anaemia. This syndrome is characterized by haemolytic anaemia, pronounced basophilic stippling of red cells, and marked increase in red cell pyrimidine-containing nucleotides (Valentine et al. 1974). P5N activity is quantitatively measured by a colorimetric method (Valentine et al. 1974) and a radiometric method (Torrance, West & Beutler 1977). For screening of P5N deficiency, the recommended method is to determine the u.v. absorption of blood extract (Beutler 1984).