Changes in the blood leukocytes have been studied in 112 opossums ranging in age from newborn to adult. Very few circulating leukocytes are present at birth, and these consist mainly of neutrophil granulocytes, with many immature forms. Leukocytes increase rapidly in number during the first 2 weeks of postnatal life, due to an accumulation of granulocytes. While all three types of granulocyte share in this early increase, initially it is the neutrophils which predominate. However, eosinophil granulocytes increase rapidly and their number soon equals or exceeds that of the neutrophils. Eosinophils remain a prominent, but lesser, component of the blood leukocytes throughout all subsequent periods of development. Lymphocytes are present only in small numbers during the first 8--10 days of postnatal life, and during this time small lymphocytes are absent from the peripheral blood. Apart from increased lobation of the granular leukocytes, the white blood cells show no remarkable cytological changes during the postnatal period.