During the course of infection of rabbits with vaccinia virus, macrophages obtained from the peritoneal cavity develop bactericidal activity and the replication of vaccinia virus becomes restricted in these cells. The abortive replication of vaccinia virus in the activated macrophages was characterized in the present study. The virus adsorbed to and was uncoated equally well in macrophages from both normal and infected rabbits. A burst of deoxyribonucleic acid synthesis of comparable magnitude took place 3 to 6 h after infection in both normal and activated macrophages. Although the production of viral antigens, as detected by immunodiffusion and immunofluorescence, was the same in both types of cells, very few virus particles were formed in activated as compared with normal macrophages. We conclude that a block in a late step of the virus replication cycle occurred in the activated macrophages.