The authors studied the changes in the percentage of reticulocytes in blood preserved for three weeks. The samples were obtained from healthy adults and stored at 4 degrees C. An automated reticulocyte counter was used for measurements. After one week, the relative percentage of reticulocytes had decreased to around 80% of the initial value, and it reached 60% after three weeks of storage. When the y-axis of plots of the change against time was converted to a log scale, negative correlation was strong (r = -0.9972), and the curve was useful for estimation of the reticulocyte count in preserved blood. Stored blood might be used as a control material for reticulocyte counting by automated methods.