The writer has investigated the nucleic acids of silkworms. In these studies, he applied the alkali treatment, which was proposed by Schmidt and Thannhauser,8 for separation of deoxyribonucleic acid (DNA) from contaminated ribonucleic acid in the crude DNA preparation of the insects. In the present paper, how this treatment affects the physical properties of DNA was investigated using bovine spleen DNA. The alkali-treated DNA was obtained as white and somewhat thready material. The viscosity of its neutral solution raised rapidly showing its repolymerization (Fig. 1). The repolymerized DNA was compared with the original intact DNA in its viscosity and sedimentation. The axial ratio of repolymerized DNA calculated in accordance with Simha's equation from its volume fraction intrinsic viscosity [η]ν (Fig. 4) was 140 and one-half of that of intact DNA (Fig. 5). The molecular weight, however, seemed to be very much larger in the repolymerized one than the intact one. The sedimentation constant S of the intact DNA was 8.40 x 10^-13 and gave molecular weight 656,000 refering to the datum in viscosity-measurement. While, the repolymerized DNA sedimented too rapidly to measure the constant. The binding power in the repolymerized DNA might be feeble, because the repolymerized DNA was easily depolymerized by warm (Fig. 3) and showed thixotropy (Fig. 2) simliarly to the observation of Zamenhof and Chargaff.