The anaemia of flexed-tailed mice is of a normocytic hypochromic type, that is, the mean cell size is normal, but the mean corpuscular haemoglobin concentration is reduced. The pathological process is confined to the embryonic mode of haemopoiesis; this persists during the first 2 weeks after birth in the anaemics, but has ceased at the beginning of the 3rd week. Pathological cells survive in the circulation for at least 2, but not more than 6 weeks. With the final disappearance of these surviving cells, the red blood picture of flexed mice becomes entirely normal. The disappearance of the anaemia takes place by the substitution of pathological by normal cells in such a way that cells of intermediate grades of abnormality are formed during the transition period.