Infection by Epstein-Barr virus (EBV), a B lymphotropic human herpesvirus, of its target cells is initiated by the binding of the viral envelope glycoprotein gp350/220 to a 145-kDa cell membrane glycoprotein (CD21, CR2) which also serves as the receptor for the complement fragment C3d (Fingeroth et al., 1984; Nemerow et al., 1987). We used the fluorescent probe 1-6-diphenyl-1,3,5-hexatriene (DPH), extremely sensitive to the polar environment, in order to analyse the membrane viscosity distribution in single cells of two lymphoid cell lines, Raji and Akata. Lipid analysis on both cell lines showed a slightly lower cholesterol:phospholipid molar ratio on Akata than on Raji cells. Measurements of cell fluidity by DPH polarization in native cells and after cholesterol enrichment indicated that the apparent Akata membrane viscosity was lower than the viscosity of Raji cells. To examine the possibility that this difference could be correlated to a difference in the behaviour of Akata and Raji cells in expressing EBV early antigens, both lines were superinfected with the EBV non-transforming P3HR1 strain. We report here evidence that lipid composition can regulate EBV entry into cells.