Two Finnish variants of reduced erythrocyte glucose-6-phosphate dehydrogenase (G-6-PD) activity were studied. The G-6-PD Espoo variant is characterized by severe enzyme deficiency which is normally non-haemolytic although primaquine sensitive. The other variant, G-6-PD Helsinki, in which the enzyme activity is moderately reduced, is associated with chronic haemolytic anaemia. The activity of the pentose phosphate pathway was not stimulated by methylene blue in G-6-PD Espoo cells, whereas in normal and G-6-PD Helsinki cells there were increases in shunt activity of 64.5- and 5.3-fold, respectively. As judged by the accumulation of 6-phosphogluconate after incubation with 6-aminonicotinamide, the activity of the pentose phosphate pathway was similar in normal and G-6-PD Helsinki cells, whereas in G-6-PD Espoo cells the metabolic flux through this pathway was decreased. Quantities of sulphydryl groups in intact cells and isolated membranes were similar in normal and G-6-PD deficient cells, as revealed by spin label experiments. In contrast to the situation in normal cells, sulphydryl groups in G-6-PD Espoo cells, and to a lesser extent in G-6-PD Helsinki cells, were sensitive to oxidation by acetylphenylhydrazine. In the G-6-PD Helsinki cells, but not in the G-6-PD Espoo cells, membrane fluidity was increased, as judged from the increased mobility of the stearic acid spin label. Mechanisms are discussed by which G-6-PD deficient cells retain adequate levels of NADPH during resting conditions, and it is suggested that the chronic haemolysis associated with G-6-PD Helsinki could be due to a defect in the lipid region of the cell membrane.