The following physico-chemical constants for the crystalline myogen A from rabbit muscle were determined: dn/dc 1.86 X lO-3 for [lambda] = 436 m[mu], and 1.80 10-8 for [lambda] 546 m[mu]. S20 = 7.86. D20 = 4.78. V = 0.735. Mol. wt.= 150,000 (136,000 by the sedimentation equilibrium method). f/fo=1.26. By different methods of observation (sedimentation, diffusion, sedimentation equilibrium) the protein was found to be homogeneous with regard to molecular wt. Within a pH range from 5 to about 9.5 the molecule is stable. Outside these limits dissociation occurs. Concentrated urea solns. cause dissociation of the molecules into halves. The molecular wt. of myogen A falls into the Svedberg multiple system.