High speed centrifugal fractionation of homogenates of rabbit skeletal muscle has led to the discovery of a soluble muscle-relaxing factor in the homogenate. Assay of the relaxing activity with deoxycholate-treated myofibrils and reconstituted actomyosin systems has established that the activity is not produced by the presence of contaminants. Relaxing activity could be removed or destroyed by charcoal, dialysis, prolonged heating, and treatment with the chelating resin, chelex-100, making it improbable that the effect is due simply to calcium deficiency. Many of the characteristics of this muscle-relaxing factor suggest that it is very similar to or the same as the factor formed by the incubation of muscle granule fractions and ATP. Evidence is presented that some soluble protein component is involved in the stabilization of the factor. The relaxing activity could be separated from the high molecular weight material in the supernatant by the technique of gel filtration. On the basis of the gel used, the molecular weight of the active agent should be less than 4000.

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