A new phosphoglycerate kinase variant (PGK New Jersey) has been purified from muscle and cultured fibroblasts of a patient with recurrent myoglobinuria. The mutant enzyme had higher than normal affinity for adenosine triphosphate (ATP) and 3-phosphoglycerate, and a shift of the pH optimum towards the acidic side. Antibodies raised against PGK purified from normal muscle were used to evaluate the presence of immunologically cross-reacting enzyme protein in tissues from the patient. Immunodiffusion and an antibody consumption test showed the presence of reduced amounts of cross-reacting material in the patient's muscle. Several PGK variants have been characterized in asymptomatic individuals or in patients with hemolytic anemia. The biochemical features of PGK New Jersey, the only known variant associated with recurrent myoglobinuria, distinguish this mutant enzyme from others.