Structural and functional characteristics of erythrocytes and isolated erythrocyte membranes from known malignant hyperthermia (MH) carriers have been examined in the hope of deriving some information concerning the underlying molecular basis of this genetic abnormality, which may represent a state of generalized membrane involvement. The increase in erythrocyte osmotic fragility which has previously been noted in porcine MH was found not to apply to the human disorder and there was evidence that in some individuals at risk osmotic fragility was in fact reduced. Although no alteration in erythrocyte membrane phospholipid profiles was detected, membrane cholesterol levels were reduced in all three definite carriers examined as well as in approximately half of the possible MH carriers investigated. No evidence for associated changes in membrane protein sulfhydryl group latency or in temperature-dependent perturbations of membrane fluidity using a stearic acid spin probe could be detected. Finally, since alterations at the level of skeletal muscle membrane -Ca++ interaction have been implicated in the pathogenesis of MH, we have examined in detail the influence of temperature on the Ca++-stimulated components of the Mg++-dependent ATPase of erythrocyte membranes from known MH carriers but no evidence of any abnormality could be found. Since MH carriers detection based solely on measurements of plasma creatine phosphokinase elevations may yield equivocal results, a decrease in erythrocyte membrane cholesterol content may provide a convenient means of identifying such individuals at risk.