Alterations in reticulocyte size occur 2–3 days after the onset of iron deficient or megaloblastic erythropoiesis and precede, by several weeks, changes in mean corpuscular volume (MCV). Iron-deficiency anemia induced in a normal subject by repeated phlebotomies was characterized by the initial development of larger than normal reticulocytes followed by an abrupt decrease in reticulocyte size. Microreticulocytes appeared 3 days after the fall in per cent iron saturation and antedated the decrease in MCV to below normal by 6 wk. Mean reticulocyte size was disproportionately smaller than normal in patients presenting with iron deficiency. In contrast, reticulocyte size increased abruptly in a patient (and rats) 2–3 days after administration of methotrexate. Mean reticulocyte size was disproportionately larger than normal in patients presenting with folate or vitamin B12 deficiency. Specific replacement therapy with iron, folate, or vitamin B12 was quickly followed by normalization of reticulocyte size.