Summary—We have observed in hepatic periovular granulomas of C3H mice infected withSchistosoma mansoni, in the chronic phase of the disease (12–19 weeks of infection), groups of early precursors and immature eosinophil granulocytes corresponding, at the ultrastructural level, to promyelocytes and myelocytes. Mitosis also seen in eosinophil myelocytes. These eosinophil myeloid foci were observed in close contact with macrophages and epithelioid cells, and they were surrounded by an extracellular matrix, rich in collagen fibres. These morphological observations give support to the concept of a peripheral proliferation of cosinophils in chronic schistosomiasis, mediated by a factor secreted by macrophages present in granulo