The use of blood volume determinations in clinical medicine has been limited by the technical difficulties, the time required, and the expense involved. The technique which will be described makes it feasible to use blood volume measurements in the same way that blood cell counts or blood chemistries are used. This communication is concerned with the reliability of this technique and its application in clinical medicine; it will not deal with the indications for or interpretation of blood volume determinations which will be dealt with in a later report. A portable and semiautomatic instrument is used to make the blood volume determinations, employing the radioisotope dilution principle. The design, electronics, mechanical characteristics, and operation of the instrument have been described in detail by Williams et al.1 The results of the in vitro and in vivo studies by these authors establish that whole blood volume and red blood cell volume