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1 Full reduction of the disulfide bridges of pure 19 S sheep thyroglobulin has been obtained by 2-mercaptoethanol in 0.1 M Tris-acetate pH 8.6 containing 8 M urea followed by alkylation (iodoacetate or iodoacetamide) of the thiol groups formed. 2 The S-carboxymethylderivative is soluble only in the presence of concentrated urea or guanidine hydrochloride. Dissolved in the presence of 5 M or 6 M guanidine at pH 7.0 or 8.6 it sediments in the ultracentrifuge as a single homogeneous boundary. 3 In 5 M or 6 M guanidine at 20° and 25° the sedimentation constants extrapolated to zero concentration are: s°20, guan 6 M= 1.2 S; s°25, guan 5 M= 2.0 S. The values for the diffusion constant and for the intrinsic viscosity are: D°25, guan 5 M= 3.4; 100 [η]20, guan 6 M= 85; 100 [η]25, guan 5 M= 78. 4 Using these values, 0.705 for v and the values of s/D determined by equilibrium or approach-to-equilibrium sedimentation, the following molecular weight have been calculated: M1= 80 × 103 or 85 × 103 (equilibrium sedimentation), M2= 81 × 103 (approach-to-equilibrium in 0.017 M SDS at pH 8.6), M3= 70 × 103 (Svedberg equation). The molecular weight computed from the values of s and [η] in 5M and 6M guanidine were 160 × 103 and 114 × 103 respectively. 5 By gel filtration on Sephadex G-200 in 0.1 M Tris-acetate pH 8.6 containing 0.017 M sodium dodecylsulphate, the S-carboxymethyl derivative of thyroglobulin is more retarded than γ-globulin 7 S and serumalbumin. After correction for protein-bound sodium dodecylsulphate the apparent molecular weight calculated for S-carboxymethyl thyroglobulin by gel filtration agrees fairly well with the molecular weight (80 × 103) computed from the hydrodynamic properties. A similar value has been obtained by gel filtration on Sephadex G-200 in Tris-acetate 0.1 M pH 8.6 containing 5 M guanidine. 6 The discrepancy between the values of the molecular weight computed from the measurements of s and [η] on the one hand and by the measurement of s/D or s and D on the other hand is discussed in relation to data in the literature. 7 It is concluded that the 12 S subunit of thyroglobulin is most probably constituted by two different peptide chains (A = B = 80 × 103) in a proportion which remains to be determined.