A mutant strain, Nagase analbuminemia rats (NAR), was established from Sprague-Dawley rats. Hematological evaluations were made on NAR of 4 to 52 weeks of age. NAR had an abnormally low number of red blood cells (RBC), a low hematocrit, a reduced hemoglobin concentration and an increased number of reticulocytes. Their plasma electrolyte level was normal. Osmotic fragility of RBC was slightly increased in the rats. Thus NAR shows a slight anemia. The in vitro experiments on RBC were performed. The incubation of blood showed a hemolytic tendency and elevated potassium efflux in the blood of NAR. In addition, an increased efflux of potassium was found in the RBC of NAR, when the RBC was washed with phosphate buffered saline and then was incubated with the saline containing CaCl2. This potassium efflux was prevented in the presence of rat albumin. These findings suggest that the deficiency of serum albumin may increase the permeability of potassium in erythrocyte membrane in NAR.