In a report of two patients who died of malignant hyperthermia, muscle adenylate kinase deficiency was identified in the father and brother of the deceased. To determine if this enzyme deficiency was a biochemical marker for susceptibility to malignant hyperthermia, we measured adenylate kinase in muscle of three survivors of malignant hyperthermia (MH) and five relatives of survivors of MH attacks with positive caffeine contracture tests. Neither the activity nor the electrophoretic mobility of adenylate kinase differed from four control values. The results show that muscle adenylate kinase deficiency is not a biochemical abnormality shared by all individuals susceptible to malignant hyperthermia.