The relationship between Cr51 red cell half-life, degree of iron deficiency anemia, and intestinal blood loss was studied in twenty-seven patients and the half-life found to be shortened in all patients with hemoglobin values under 6.5 g per 100 ml, irrespective of intestinal blood loss. Treatment of the anemia corrected the half-life. Sequestration of the iron-depleted red cells by the enlarged spleen was demonstrated in two severely anemic patients.