Viscosities of aqueous solutions of DNA preparations which had been isolated from acetone-dried cells of amylase-producing bacteria by several procedures were estimated. In general, viscosity had no relation with the fibrous appearance or powdery one of preparations. The high-DNA preparation made a solution of much higher viscosity than that from preparation of low-DNA content. However, in some cases, the viscosity of the later was higher than that of the former. This observation suggested that certain contaminants in preparation also contributed to the viscosity. As usual, viscosity was reduced more or less by adding NaCl as well as by dilution. The effect of salt was maintained even in 10^-4 or 10^-5M of final concentration, regardless of the viscosity of aqueous solution of DNA preparation. The rate of decrease of viscosity by NaCl tends to be much higher in the solution of higher viscosity than in that of low viscosity. The viscosity due to the contaminants cited above was presumed to be reduced by NaCl. Similar reduction of viscosity was also observed by KCl, NH_4Cl, CaCl_2, or MgSO_4. Of course the viscosity was reduced usually much effectively in higher concentration of salts. However, in the case of MgSO_4 and CaCl_2, the viscosity was the lowest in 10^-2M of final concentration. Some buffers, such as Tris, acetate, phosphate, A-T or even HCl and NaOH, also had the same ability to reduce the viscosity. These experiments must be called especially when we use the preparation obtained for some object, for example the substrate of the viscosimetric estimation of DNase activity.