Studies on LS-protein, the soluble double antigen of vaccinia, and on the degradation products L'S and L''S' have been made with electrophoresis and in the analytical ultracentrifuge. LS, which is homogeneous electrically and in the ultracentrifuge, has an isoelectric point at pH 4.8. At 4°C. its partial specific volume is 0.72 cc./gm., and its diffusion constant is 1.50 x 10–7 cm.2/sec. The sedimentation constant is 6.35 at 20°C., the molecular weight is 214,000, and the molecule appears to have an elongated ellipsoidal shape with an axis ratio of 1:20. L'S and L''S' are homogeneous electrically but not in the ultracentrifuge, L''S' being extremely polydisperse.