Phosphoglycerate kinase was purified from a number of tissues obtained at autopsy from a subject with the deficiency. Properties of the mutant enzyme were compared to those of PGK purified from tissue obtained from normal subjects. The purified enzyme from the propositus contained two components, a minor fraction (about 2-5%) which behaved similarly to the normal enzyme during the purification procedure, and a major fraction (greater than 95%) which could not be purified by the same procedure. The major fraction demonstrated a number of other properties which differed from the normal. These included a tendency to form aggregates, increased heat sensitivity and altered nucleotide substrate specificity. The smaller fraction appeared to have effectively identical properties to that of the normal enzyme. In keeping with its X-linked mode of inheritance, phosphoglycerate kinase from all normal tissues appeared to have identical kinetic properties, although evidence for minor variations, presumably due to post-translational modifications, was obtained.