An acute eosinophilopoiesis occurs in mice secondarily exposed to Ascaris suum, the marrow eosinophils increasing from approximately 5% to 45%, and blood levels from 50/ml3 to more than 1000/ml3, within 14 days. Eosinophilopoiesis occurs in hematopoietic compartments of the bone marrow and is associated with four other cell types: branched stromal cells, macrophages, lymphocytes, and reticular cells adventitial to venous sinuses. Branched stromal cells, a newly recognized cell type, are presented in intensified hematopoiesis of at least several blood-cell types. They are characterized by extensive branches surrounding contiguous hematopoietic cells, dense cytoplasm, and multiple communications between nuclear cisternae and an extensive, dilated endoplasmic reticulum. These cells, moreover, have a capacity to coalesce to form an extensive, branching, multinucleate giant-cell system. They are neither phagocytic nor fibroblastic, and attend every phase of eosinophilic hematopoiesis. Macrophages lie among eosinophils, and they possess many processes which extend among eosinophils. The may constitute, with the eosinophils, islets similar to erythroblastic islets; or they may lie among eosinophils in sheets. Macrophages also lie against the outside surfaces of vascular sinuses and extend transmural processes into the lumina. Lymphocytes of a type not yet known are regularly present among developing eosinophils. Adventitial cells of vascular sinuses, fibroblastic cells, extend processes deep into perivascular hematopoietic spaces, and thereby envelop eosinophils and other maturing blood cells. While adventitial cells can be dense, they were typically quite lucent and had microfilaments clustered beneath their plasmalemmae. With the large-scale blood-cell delivery characteristic of this model, marked changes occurred in the walls of the vascular sinuses. Adventitial cells moved away from the vascular wall, permitting blood cells direct access to the basal surface and reducing their cover from more than 65% to less than 20%. Both adventitial and endothelial cells may be compact and dark, or expanded and quite lucent. Heterolysomes occur in moderate-to-large numbers in both adventitial and endothelial cells, and large gaps filled with blood cells in transit occur in endothelium. The hematopoietic-associated cells and the vasculature described here would appear to provide the cellular microenvironment which regulates hematopoiesis.