Vaccinia virus-induced morphological lesions were studied in LLC-MK2, HeLa and L cells. In LLC-MK2 cells, cell rounding occurs within 30 to 60 min after infection with 300, 900 or 2700 particles/cell and the time of appearance of these changes is dependent on the multiplicity of infection (m.o.i.). When cycloheximide (300 microgram/ml) is added to cultures at the time of infection, early cell rounding is prevented regardless of the m.o.i. However, cell rounding does occur when cycloheximide is removed, and its time of appearance and extent depends upon the time of removal of the compound and the m.o.i. Upon removal of cycloheximide at I or 2 h after infection early cell rounding occurs, and virus polypeptide synthesis is evident in cells infected at all three multiplicities. However, when the drug is removed at 4 hr after infection, cell rounding and virus polypeptide synthesis occur only in cultures infected at 300 particles/cell. Early morphological changes are also prevented in HeLa and L cells infected at 300 particles/cell in the presence of cycloheximide. These changes occur only if the compound is removed up to 2 h after infection in HeLa cells and up to 40 min after infection in L cells. Early morphological lesions are not seen if the compound is removed at later times. The occurrence of early morphological changes in HeLa and L cells is also correlated with the synthesis of virus polypeptides. All cell types, when infected at 2700 particles/cell in the presence of cycloheximide, or inhibitors of RNA synthesis, display cell fusion. Thus, whereas early morphological changes require virus protein synthesis to become manifest, cell fusion occurs in the absence of virus RNA or protein synthesis and may be mediated by a component of the virion.