An X-chromosome linked phosphoglycerate kinase deficiency in erythrocytes and leucocytes was discovered in a large German kindred. Seven males of two generations were found to have only 21% of the normal enzyme activity in their erythrocytes, and twelve females of three generations showed various degrees of this defect. The differences in the expression of the deficiency in heterozygote females are explained by the Lyon hypothesis. The deficiency is caused by a variant enzyme, named phosphoglycerate kinase München. Although it differs from the normal enzyme electrophoretically, the two enzymes resemble one another closely in many respects. They have essentially the same Km for the substrates of the backward reaction, identical pH optima and similar rates of thermal inactivation. In contrast to the nine previously described phosphoglycerate kinase deficiencies, all of which are associated with haemolytic anaemia, the carriers of phosphoglycerate kinase München show no overt clinical symptoms. The erythrocyte concentrations of adenine nucleotides and 2,3-diphosphoglycerate are normal.