Abstract. Hemograms, erythrocyte volume distribution curves, serum iron and percent transferrin saturation determinations were done on samples from 50 kittens to characterize feline erythrocytic responses between one and ten weeks of age. At one week of age, all kittens had marked macrocytosis and anisocytosis. Most of the macrocytic cells no longer were present by five weeks. Macrocytes were replaced by erythrocytes of normal volume in 30% of the kittens. At five weeks of age, 70%of the kittens had produced microcytic cells which correlated with significantly lower packed cell volume values (p < 0.02) and mean corpuscular volume values (p < 0.00I). Byseven weeks of age, kittens with microcytosis were producing normocytic erythrocytes. Between two and four weeks of age, kittens with microcytosis had significantly lower serum iron and percent transferrin saturation values (p < 0.01) compared to kittens without microcytosis. Between five and seven weeks of age, all kittens had very high iron values which were significantly greater than those of healthy adult cats (p < 0.0 I). Six kittens with low iron values received an iron dextran injection at two to three weeks of age. At five weeks, these kittens had significantly greater mean corpuscular volumes, packed cell volumes (p < 0.0 I) and lower percentage of microcytic cells (p < 0.00I) than littermate controls. These data demonstrate that transient microcytosis and anemia observed in kittens is attributable to iron deficiency. It also was demonstrated that erythrocyte volume distribution curves were more sensitive than the mean corpuscular volume in detecting microcytosis in kittens.Anemia, attributable in part to transient iron deficiency, occurs in the young of numerous species in association with rapid growth rate and all milk diet [I, 6-9, 12, 13, 16,21]. This anemia can be reduced or eliminated by neonatal iron supplementation [6-8, 10, 14-16,21,24]. Hematologic studies in kittens have revealed a mild to moderate normocytic anemia between four and eight weeks of age which slowly disappeared between three and six months of age [2,20]. It was suggested but not documented that the anemia was due to iron deficiency. These studies also indicated that mean corpuscular volume was high at birth and slowly decreased to adult values by about six months of age. No microcytosis was observed. Using sequential erythrocyte size distribution curves (erythrograms), we have observed considerably different changes in erythrocytes. Erythrograms are much more sensitive than mean corpuscular volume values in detecting disturbances of erythrocyte size [3,4,17]. In I