Abstract The DNA of murine cytomegalovirus (MCV) was analyzed, in comparison with T4 and herpes simplex (HSV) DNAs, by velocity sedimentation in neutral and alkaline sucrose gradients; and by centrifugation to equilibrium in cesium chloride. The molecular weight of MCV-DNA was calculated to be 132 × 10 6, relative to 110 × 10 6 for T4 DNA, and 85 × 10 6for HSV DNA. Like other herpes viruses, the MCV genome possessed alkali sensitive single strand regions. However it was possible to obtain some of the MCV DNA molecules free of alkali sensitive regions. Intact MCV DNA banded in cesium chloride as a homogenous collection of molecules of density corresponding to 59% G + C. However, quarter molecules or smaller fragments banded at two discrete densities, corresponding to 57.5 and 61.5% G + C.