AS WE all know, rubella would be of very little concern to us if it were not for the effect the virus has upon the fetus. However, minor disease that it is, it still is associated with certain more severe manifestations or complications such as arthritis, encephalitis, and thrombocytopenia. These are of some interest in themselves, but it becomes particularly desirable to understand their pathogenesis, frequency, and other features as we consider the use of living vaccines. The same complications that occur with the natural illness must be looked for in the vaccine trials. Although the vaccine virus may have lost some attributes in that it produces a disease even milder than the natural one and less communicable, there is no assurance that there will be any reduction in frequency or severity of complications. Were we better informed about the pathogenesis of the various disease manifestations, it might be possible