We have used laser beat frequency light scattering spectroscopy to measure, at several pH values, the electrophoretic mobilities of purified avian myeloblastosis (AMV), murine leukemia (MuLV), murine mammary tumor (MuMTV), and feline leukemia (FeLV) viruses. The mobilities of these viruses are similar at pH greater than or equal to7 (-2.7 to -3.2 X 10(-4) (cm/sec)/(V/cm). The isoelectric points of MuLV and AMV are apparently less than pH 3, whereas for FeLV the data could be interpreted to indicate an isoelectric point between 3 and 5. Using a Debye-Hückel model to describe the interaction between electrolytes and virus, we show that our values for the mobility of MuMTV, obtained in ionic strength 0.005, are consistent with the values of Sarkar et al. ((1973), Cancer Res. 33, 2283), obtained in ionic strength of 0.10. This model is then used to calculate surface charge densities. In terms of the density of charged groups, the RNA tumor virus envelope is not very different from the erythrocyte membrane.