A further attenuated strain of measles virus, called AIK strain, developed by Makino and his colleagues at the Kitasato Institute, Tokyo, was adapted to human diploid cells (HDC), MRC-5. In a field trial it was found that this strain initiates a low rate of clinical reactions and 100% seroconversion in vaccines. An attenuated strain of rubella virus, called Takahashi strain, isolated by Shishido at the National Institute of Health, Japan, was also adapted to HDC, MRC-5. Brief details of production of an experimental rubella vaccine are given. Up to 20 harvests of virus were made from each culture without any apparent alteration of diploid cells.