Malignant hyperthermia (MH) is a pharmacogenetic disorder provoked by volatile anesthetics and depolarizing muscle relaxants. The preoperative diagnosis of MH is difficult because it requires a large muscle biopsy and a laboratory dedicated to such diagnostic studies. The authors performed electrophoresis of six muscles taken from MH patients or their relatives to determine whether the protein composition is different from normal muscle. MH muscle was found to contain large amounts of two low molecular weight proteins (15,000 daltons and 13,500 daltons) that are not present in normal muscle. Although it has not been determined that these differences are specific for MH, this finding eventually might be of assistance in diagnosing MH.