The mechanism of infection and of multiplication of Coxsackie B(5) virus was studied using dog kidney cells (to be abbreviated CS cells). As the result it has been found that Coxsackie B(5) virus has a high susceptibility to CS cells and proliferates by demonstrating a specific cytopathic effect. The results may be summarized as follows. 1. In the infection pattern of Coxsackie B(5) virus to CS cells, the virus is found to proliferate with a high susceptibility to thecells. In addition, morphologioal changes of the cell and the course of virus multiplication parallel relatively well with each other. Namely, both cell degeneration and the virus infectivity reach to their peak on 72 hours after infection, and the infectivity decreases along with gradual detachment of the cells in cultures. 2. In one-step growth experiment conducted after simultaneous infection of a large number of CS cells with increasing amounts of virus, the cycle of the one-step multiplication after a eclipse phase (4-6hr) is found to be about 16 hours and it is assumed that the cycle is repeated following the infection. Furthermore, it is presumed that the release of regeneratad virus into liquid layer commences within 7 hours and it proceeds very rapidly along with the proliferation of the intracellular regenerated virus.