THIS communication reports the determination of the molecular weight of rabbit tropomyosin by equilibrium sedimentation in a solvent consisting of 8 M urea, 0.2 M sodium chloride, 0.025 M sodium dihydrogen phosphate adjusted to pH. 7.0, and 0.1 M β-mercaptoethanol. Columns 1 mm high were used and the speed was adjusted so that the weight-average molecular weight could be calculated from the refractive index gradient at the mid-point of the column1,2. The results are given in Fig. 1 as a plot of 1/Mapp versus concentration, expressed as Δn, the refractive index increment. For the molecular weight calculations a partial specific volume of 0.728 c.c./g was used, the figure determined by Kay8 for tropomyosin in 8 M urea. A least-squares line through the points extrapolates to a molecular weight of 34,000.