1. 1. An examination has been made of the sedimentation and electrophoretic behaviour of separate and mixed solutions of a thymus deoxypentose nucleic acid (DNA) preparation and of the same DNA degraded by sonic irradiation. 2. 2. In sedimentation at an ionic strength of 0.3, the presence of 10% of the degraded DNA in a mixture could be detected, but the method was limited by self-sharpening effects in the boundary and by the insensitivity of sedimentation coefficient ( s) to molecular weight. It is concluded that the original DNA cannot contain as much as 10% of such degraded material, but there is some indication of polydispersity in sedimentation at low concentrations. During sedimentation of a mixed solution containing equal amounts of the two DNA's the area of the schlieren peak of the slower, degraded DNA was enhanced by the operation of the Johnston-Ogston effect. 3. 3. In electrophoresis, the individual DNA samples had the same mobility, in spite of the very great difference in the viscosity of their solutions. No separation was observed with the mixtures. 4. 4. Solutions of the degraded DNA (ionic strength = 0.3) had a very much lower viscosity than those of the original DNA and behaved as Newtonian liquids. This enabled its intrinsic viscosity to be determined and a molecular weight for the degraded DNA of the order of 3–4·10 5 was calculated on the basis of different models. 5. 5. The sedimentation data are discussed with reference to the sedimentation of long rod-like] molecules in general. Application of Perrin's equation shows that, for very large axial ratios, s is chiefly dependent on the minor axis of the equivalent prolate ellipsoid. If this model is assumed, the anhydrous diameter of the undegraded DNA molecule in solution can be estimated as 25 A. The data are also discussed in terms of a flexible chain model, and, on that basis, a molecular weight of about 8.5·10 6 is obtained.