Immunoelectron microscopy using an anti-VP2 monoclonal antibody complexed to colloidal gold has been used to study the mechanism of bluetongue virus (BTV) release from infected cells. Examination of the BTV-infected cell surface revealed that viruses are released both as enveloped particles, by budding through the plasma membrane, and as nonenveloped particles by "extrusion" through the membrane. Particles being released and those remaining on the cell surface retain an association with the cortical layer of the cytoskeleton. Analyses of virus particles released from infected cells and the intracellular viruses in the cytosol and attached to the cytoskeleton indicate that although the three populations have similar particle to infectivity ratios they differ in their ability to bind gold-labeled anti-VP2 antibody. The fact that released viruses bind less antibody than intracellular viruses suggests that virus release from infected cells may be associated with either a loss of VP2 or a rearrangement of the virus outer coat which obscures a proportion of the reactive epitopes on the virus surface. Electron microscopic observations also indicated that, in addition to virus release, events at the plasma membrane resulted in the uptake of progeny virus by endocytosis. Elevation of intraendosomal/lysosomal pH by lysomotropic bases and an acidic ionophore inhibited BTV replication when added to cells concurrently with the virus. Addition of such agents to infected cells at 4 hr p.i. decreased both the maximum titer of released virus and the rate at which virus antigen was synthesized in infected cells. Addition of anti-BTV antiserum 4 hr p.i. also resulted in a decreased rate of intracellular virus antigen accumulation. These results suggest that superinfection of BTV-infected cells by progeny virions effectively increases the multiplicity of infection and enhances the kinetics of BTV replication.