The authors describe a safe, simple, sensitive, and accurate technique of measuring moderate amounts of blood loss using erythrocytes labeled with indium-111. Donor red cells were labeled with indium-111 and tracer amounts infused into dogs. The magnitude of red cell loss was determined by the difference between two sequential volume measurements. The coefficient of variation of sequential measurements of the same red cell volume was 4.6 +/- 1%. The sensitivity of the technique, as determined by comparison of measured and estimated volumes, was 20.5 ml. The measured volume was related linearly to the calculated volume of red cells lost (r = 0.98). The authors recommend indium-111-labeled erythrocytes as a means of obtaining multiple measurements of small to moderate amounts of hemorrhage.