EB virus genome-carrying B-cell lines have been double-stained for B-cell activation or maturation antigens and viral antigens using indirect immunofluorescence. The vast majority of cells express activation antigens, whereas the plasma cell antigen PC1 is present on only a few cells. Staining for this antigen doubles with staining for EB viral capsid antigen. Thus it marks cells in the virus replicative cycle, and these cells are negative for the EB virus nuclear antigen EBNA. We suggest that within EB virus genome-carrying B-cell lines the maturation of a few cells to the stage of expression of plasma-cell markers leads to the loss of EBNA expression and consequent loss of the immortalized state of the cell. This would allow completion of the lytic cycle with the production of virus particles and cell death. The implications of these findings are discussed.