Single-channel recordings have indicated that ryanodine receptor (RyR1) mutation Arg615Cys of porcine malignant hyperthermia-susceptible (MHS) muscle is not directly associated with the enhanced caffeine sensitivity of MH(S) muscle [1]. In the present study, the effect of a novel activator of RyR1, 4-chlorom-cresol (4-CmC), was investigated on high-affinity [3H]ryanodine binding to porcine skeletal sarcoplasmic reticulum. The 4-CmC affinity of [3H]ryanodine binding to MHS vesicles was 2-fold higher compared to that in normal tissue. This enhanced affinity was confirmed when the effect of 4-CmC on [3H]ryanodine binding to the isolated CHAPS-solubilized MHS RyR1 was investigated. 4-CmC is, therefore, suggested to be a potent tool to distinguish between Ca2+ release from MHS and normal muscle.