SynopsisA Malvern laser light-scattering instrument has been modified for use at scattering angles down to 5O and both total intensity and quasi-elastic scattering experiments. A sample of sheared, length-fractionated calf-thymus DNA was characterized by sedimentation, viscosity and electron microscopy. Quasi-elastic scattering and absolute intensity determinations were performed with the laser instrument and intensity determinations only with a Fica conventional light-scattering photometer. The total intensity experiments gave Mw = (3.75 f 0.15) X lo6 and (R2);'' = (206.9 f 10.3) nm which yielded a value for the persistence length, allowing for polydispersity, of 66 f 6 nm. The quasi-elastic experiments at scattering angles below 20" gave D&,,, = (2.23 f 0.06) X cm2/sec which combined with S $ , = 15.6 in the Svedberg equation gave Mu = (3.73 f 0.18) X lo6. In addition, from the higher angle data we extracted a value of the longest intramolecular relaxation time, T I of 17.5 msec. This is not in particularly good agreement with T I predicted by the Zimm-Rouse theory using our other experimental parameters. The disagreement may be due to the restricted applicability of the Zimm-Rouse spring-bead model as a quantitative representation of DNA molecules. Alternatively, it may be due to present difficulties in the unambiguous interpretation of molecular motions from the experimental autocorrelation functions.