SummaryRabbitpox virus (RPu+), 18 white pock (u) mutants derived from it and several strains of vaccinia virus were examined in respect to their cytopathology in chick embryo fibroblast cultures, and several continuous lines of mammalian cells; PK‐2a (pig kidney), L929 (mouse fibroblast), FAF (Chinese hamster fibroblast), AT (Chinese hamster, epithelial), KB (human epithelial), FL (human amnion) and Chang liver (human epithelial). The plaquing characteristics of the viruses, and the capacity to multiply exhibited by those which did not produce plaques, were investigated in chick fibroblast, PK‐2a, and L929 cells.Cytopathic changes were characterized by early toxic effects, cellular fusion, and progressive destruction of monolayers, the pattern of changes being a function of specific virus‐cell combinations. With input inocula of 1‐5 PFU per cell, most of the virus strains and mutants produced toxic changes in most of the epithelial type cells (PK‐2a, FL, Chang liver) but not in the fibroblastic cells, and to a very slight degree in KB cells. Three mutants which failed to multiply in PK‐2a cells nevertheless produced moderate toxic changes in them.Pronounced fusion of cells was produced in the epithelial type cells, but not in fibroblastic cells, by most of the viruses which multiplied in them, but not by mutants which failed to multiply.All the mutants produced plaques on chick embryo fibroblasts, and these could be distinguished as turbid or clear, and differed in size and in cultural requirements for maximal plating efficiency. A standard plaque assay method is described which gives good plaques with the majority of mutants.Conditions for optimal plaque assays on PK‐2a and L929 cells are also described, and several distinctive plaque types were recognized. Some mutants failed to produce plaques on PK‐2a or L929 cells, and in all cases except one this was shown to be due to their failure to multiply in the cells concerned.