The Oka strain of varicella virus, isolated in our labolatory, was serially cultivated in guinea-pig embryo cultures (GPEC), and a considerable amount of cell-free virus was obtained from infected cell. GPEC passage virus at the 6th passage level was used in a small scale field trial. Susceptible children of 1 to 10 years old were injected subcutaneously with 100 to 1,000 PFU of virus. No clinical reactions due to the vaccination were observed in any children, and a high rate of antibody response was obtained with viral doses of more than 200 PFU. Attenuated virus obtained by passage in GPEC was propagated in human diploid (WI-38) cells, and it was also effective in inducing an immune response without clinical reactions. The results show that the Oka strain of varicella virus passaged in GPEC and human diploid (WI-38) cells may be used safely and effectively as a live attenuated vaccine.