Ultrastructural and functional studies were carried out on nurse shark (Ginglymostoma cirratum) peripheral blood cells in order ot identify cells of definitive morphology and specific function. Along with erythrocytes and thrombocytes, four morphologically distinct leucocytes are recognized in peripheral blood: two types of granulocytes, the 'eosiniphil' and the 'granulocyte', and two mononuclear agranulocytic cells, one resembling mammalian macrophage and monocyte, the other resembling mammalian lymphocyte. Also present in peripheral circulation are blast-like cells and mitotic cells. In vitro phagocytosis was demonstrated by the monocyte-macrophage and the granulocyte while thrombocytes, eosinophils and lymphocytes showed no phagocytic activity in the system studied. It is stressed that care must be used in drawing functional analogies between blood cells of a mammal and an elasmobranch on the basis of morphological similarity alone.