The reaction between Pichinde virus and homologous antisera has been studied using a plaque size reduction method. The incorporation of antiserum in the overlay of infected Vero cell monolayers revealed a pattern of virus-cell interactions which were manifested by both a significant reduction in the diameter of virus plaques, and regeneration of cells in the centre of each. Electron microscopy demonstrated that antibody molecules were bound to virus particles budding from the surface of infected cells resulting in the formation of extracellular virus-antibody complexes. These aggregates were subsequently detected in vacuoles of freshly-infected cells. In the absence of virus neutralization, reaction of Pichinde virus with homologous antiserum leads to the formation of infectious aggregates which due to their larger size restrict the rate of plaque development.