The effects of increasing concentrations of methylmercuric hydroxide (CH3HgOH) on the rate of sedimentation, So, and intrinsic viscosity, [q], of T7 DNA have been studied at 20Β°C in 0.005,0.05, and 0.5M NapS04, respectively, whereby each salt solvent contained, in addition, 0.005M sodium borate, pH 9.18, as a buffer. Both So and [q] are independent of organomercurial concentration as long as DNA remains native. Denaturation, brought about by the complexing of CH3HgOH with the polymer, produces large changes in So as well as [q]. The sedimentation coefficient increases strongly with increasing organomercurial concentration once strand separation has occurred. Experimental difficulties prevented measuring of [q] in the posttransition region. The data on SO have been used, in combination with available information on the so-called density increment (dp/dc2):, to obtain information on the frictional properties of single-stranded and methylmercurated T7 DNA. The frictional coefficient, defined as ( 2 = Mp(dp/dc2):/S0q,N~, where Mp is the molecular weight of T7 DNA, c p is the concentration of DNA in g/ml of solution, qr the relative viscosity of the salt solvent, and NA is Avogadro's number, was evaluated for all three salt media as a function of organomercurial concentration. ( 2 of native T7 DNA was found not to be sensitive to changes in ionic strength; but f p of single-stranded and methylmercurated T7 DNA varied strongly with salt concentration. Since fz of single-stranded T7 DNA was barely affected by organomercurial concentration at a given ionic strength, it is concluded that the dramatic variations of So with pM (pM = -log[CH3HgOH]) observed in the posttransition zone reflect only changes in the thermodynamic interactions ("preferential interactions") existing between DNA and the various other solution components, but not changes in the coil dimensions of the polymer.