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Osmotic pressure measurements are reported for reduced protein polypeptide chains in 6 M guanidine hydrochloride. Previous results have indicated that the polypeptide chains are random coils, devoid of longrange structure, in this solvent medium, and end-to-end distances have been calculated for them from viscosity measurements. In this paper these results are corrected to unperturbed dimensions with the aid of second virial coefficients obtained from the osmotic pressure data. The relation between the unperturbed end-to-end distances (in A) and the number («) of amino acids per chain is found to be (¿2)0 = (60 ± 10)n, a result essentially identical with that obtained earlier from viscosity data alone by use of the Kurata-Stockmayer procedure. These dimensions are somewhat shorter than would be predicted on the basis of theoretical and experimental studies of Miller, Brant, and Flory, but the discrepancy is judged to be within the limits of error inherent in the experimental and theoretical procedures.

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