The head-kidney of the sea bass is a source of erythropoietic and thrombopoietic cells. No significant lymphopoietic activity is found. Erythropoiesis, thrombopoiesis and granulopoiesis are intermingled, which suggests that only one environmental "niche" exists to modulate the lineage development. No numerous proerythroblasts are present. Erythropoiesis consists of a proerythroblast, basophilic erythroblast, polychromatophilic erythroblast, acidophilic erythroblast, young erythrocyte and old erythrocyte, the latter being nucleated and without nuclear vacuolation. The numbers of the free ribosomes and polyribosomes decrease progressively from proerythroblasts to young erythrocytes, suggesting a high synthetic activity from early on. Immature erythropoietic cells show pits and protrusions correlated to micropinocytotic vesicles, indicating ropheocytosis. The peripheral band of microtubules is the most remarkable cytoplasmic structure in proerythroblasts. Granular cytoplasmic inclusions or lysosomes were not observed in erythropoietic cells. The old erythrocyte reveals an electron-dense homogeneous cytoplasm with occasional mitochondria and a small Golgi apparatus. Thrombopoietic cells comprise both the immature and mature prothrombocyte and adult thrombocyte. Nuclear and cytoplasmic densities increase and the surface connected canalicular system develops during maturation. A marginal band of microtubules is present in the cytoplasm from prothrombocytes to mature thrombocytes. Some pseudopodial processes, dense granules and vesicles, probably indicating passive storage, are also observed.