1. The partition coefficients of some different preparations of nucleic acid from different sources have been determined in the polymer phase systems dextran: methylcellulose: water and sodium dextran sulphate: methylcellulose: water. 2. For nucleic acids of different molecular sizes, the negative logarithm of the partition coefficient was found to increase nearly proportionally with increasing sedimentation coefficient. 3. In the counter-current experiment with thymus DNA, the distribution after each transfer changed in such a way that, according to the above-mentioned relation between sedimentation and partition coefficients, the low molecular fractions of the heterogeneous DNA moved with the top phase. Soluble RNA has also been separated from a mixture with thymus DNA and the separation was checked with a pyrimidine analysis. 4. Thymus DNA has been fractionated by the counter-current technique. After a few transfers, analysis in the ultracentrifuge showed a fractionation of the material and each tube contained DNA with a more homogeneous sedimentation coefficient distribution—the higher the tube number, the lower the coefficient. 5. In most systems, an interface layer of nucleic acid is formed. This can be used to obtain a more homogeneous thymus DNA. 6. The adsorption of nucleic acid at the interface can also be used as an analytical tool. A few per cent of high molecular RNA, in a preparation of soluble RNA, can be detected by the formation of an interface layer in a suitable polymer phase system. 7. Contaminated DNA from phage T2 was shown to deviate strongly from the main relation between the sedimentation coefficient and the partition coefficient. When the preparation was dialysed, and thus freed from parts of the contaminating substances, which are probably amines, the negative logarithm of the partition coefficient increased towards a better agreement with the main relation.