There is no single, simple diagnostic test available to enable identification of malignant hyperthermia (MH) susceptible individuals. Recently, two novel low-molecular-weight proteins (15,000 daltons and 13,500 daltons) that were not present in normal muscle were identified in MH muscle and it was felt that this might eventually be of assistance in diagnosing MH. The authors of this report have been unable to verify these results. Polyacrylamide gel electrophoresis of the soluble proteins from muscle of four MH-susceptible and four normal individuals showed no differences in the electrophoretic fractionation patterns. Therefore, the authors conclude that the differences in protein composition previously reported in MH muscle are not characteristic of this syndrome.