Summary A study was made of the mechanism of microfocus formation and of the nature of the inclusion bodies that appear in the cytoplasm of RK-13 cells infected by rubella virus. Direct passage of virus particles from one cell to another, at least in the early stages of the infection, is posed as a possibility, and the lysosomal nature of the cytoplasmic inclusion bodies is demonstrated by histochemical methods. A description is given of conditions required in the monolayer for the formation of a microfocus, which is the characteristic manifestation of the c.p.e. of the rubella virus in this cellular system.