A new variant of red cell glucose-6-phosphate dehydrogenase (G6PD) has been found in a Caucasian man with congenital non-spherocytic haemolytic anaemia. This variant has reduced activity, increased thermolability, increased Michaelis constants for glucose-6-phosphate and NADP, slightly increased electrophoretic mobility, and a biphasic pH-activity profile. The red cell adenine compounds and ATP, are in normal limits. The increased activity of red cell NADP-glutathione reductase is probably the expression of a mechanism of compensation for the decrease of G6PD and a consequence of the decrease of NADPH.