Enzyme and oxygen transport studies have been made on seven members of a kindred with red cell deficiency of phosphoglycerate kinase (PGK), an enzyme known to be linked to the X-chromosome. The two affected males were children, had little detectable PGK activity and had severe neurological damage. Both had moderate haemolytic anaemia complicated by the occurrence of haemolytic crises. Red cell ATP levels were substantially decreased in both subjects. However, they had more than double the normal level of 2,3-diphosphoglycerate (2,3-DPG) in their red cells, associated with a rightward shift in their oxygen-haemoglobin equilibrium curves. This shift in the curve was sufficient to permit oxygen delivery to most body tissues, including the brain, at better tensions than normal, except during a haemolytic crisis. Two of the three known heterozygotes were aged over 70. They had moderate haemolytic anaemia, considerable reduction of red cell PGK activity (19% and 35% of normal, respectively), and considerable increase in red cell 2,3-DPG with rightward shifting of the oxygen-haemoglobin equilibrium curve. The other known heterozygote was aged 22 and showed little abnormality in her blood oxygen transport.