We have demonstrated that the small plaque (SP) variant of Sindbis virus is more extensively neutralized by complete antibody and serum IgG fractions than is the large plaque (LP) variant. The Fc portion failed to demonstrate any neutralizing potential and did not block subsequent neutralization by IgG or Fab fragments. The LP and SP variants of Sindbis gave identical patterns upon isoelectric focusing. Treatment of the virus with intact serum IgG or isolated Fab fragments was found not to have an irreversible effect on the net charge of the virus population. The selective processes involved in the maturation of the nucleocapsid implicate the incorporation of charged host determinants which are then reflected in the charge of the virion. It is suggested that the virus core has a preferential affinity for those components of the plasma membranes which have an isoelectric point at pH 3.6.