Blood volume studies were performed in 18 healthy men and 564 male patients with a variety of clinical diagnoses. Although the plasma volume measurements obtained with iodinated I 125 serum albumin were accurate in patients with erythrocytosis and in healthy patients, this method overestimated the plasma volume in patients with traumatic injuries, carcinoma, cardiopulmonary disorders, and miscellaneous diagnoses. However, in 47 patients with traumatic injuries, carcinoma, and miscellaneous diagnoses, cold agglutinin labeled with radioactive iodine (125I) gave an accurate measurement of plasma volume. These data demonstrate the limitations of the iodinated albumin molecule in measuring plasma volume and in indirectly estimating red blood cell (RBC) volume from the total body hematocrit value and the plasma volume as determined with iodinated I 125 serum albumin. The best method was by determining red blood cell volume with radioactive chromium (51Cr)-labeled RBC, and estimating plasma volume.