Data are presented to show that in aplasia of the bone marrow, plasma iron turnover should be expressed per total expected hemoglobin mass, rather than per blood volume. Poor correlation between plasma iron turnover and erythrocytic iron utilization occurs in conditions in which the total bone marrow activity is less than normal. Under these conditions it seems that erythrocytic iron utilization is a better index of total erythrocyte production than plasma iron turnover, or the 59Fe T, clearance can be used as a guide.