In 8 emergency care patients blood volume was determined using Cr5I labelled erythrocytes and indocyanine green (ICG). Prior to measurement of ICG in blood with a spectrophotometer, the blood was hemolyzed with Triton-X. A close correlation of r = 0.97 between the Cr51 and the ICGTR-X estimates was found; the ICGTR-X volume was about 3% lower than the Cr51 volume. In five additional in vitro experiments the ICGTR-X method was found to reflect real volumes with an insignificant error of less than 1%. Blood volume determination with ICGTR-X cannot be applied in cases of circulatory failure. ICG should be administered in a dose of 0.5 mg/kg of body weight. For calibration purposes, a two point calibration curve (point 1: point of intersection of x and y axis; point 2: 5 mg ICG/1 of blood) is sufficient. From these preliminary experiments it is concluded that the ICGTR-X method is a rapid and simple technique of blood volume determination with multiple reproducibility which can be carried out in any clinical laboratory.