Changes in the blood cells have been considered as an aid in the diagnosis of rubella hut there has been no general agreement on the differentiation of the changes seen in rubella from those seen in measles and other disorders. Observations made during an epidemic of rubella in the Falkland Islands supplemented by material from London hospitals, afforded an opportunity for further study of the blood changes in rubella. The author believes the following changes to be indicative of rubella: (1) Monocytes, Turk cells and (especially) plasma cells, abundant. (2) Lymphacytes and neutrophil leucocytes, reduced. (3) Stab-cells, little increased. (4) Metamyelocytes, scarce. (5) Degenerate forms of lymphocytes, present. These changes in the blood linger long after the attack of rubella. The distinction between the changes in rubella and those seen in measles and infectious mononucleosis is discussed. The author is of the opinion that the changes in the blood are sufficiently characteristic to be an important aid in the diagnosis of rubella, and that this is particularly advantageous when it is necessary to make a correct diagnosis in order to provide protection for an exposed pregnant woman. The characteristic changes in the blood were found in all cases of rubella examined between the first and ninth days of illness.