THE PHENOMENA of erroneous anemia and polycythemia as related to blood volume have been given little attention. The red cell mass and circulating hemoglobin may be entirely normal in a person in whom the peripheral blood elements and hemoglobin concentration are below the accepted normal values. Pseudo-polycythemia may be caused by a decreased plasma volume. Even in polycythemia vera there may be a lack of correlation between the peripheral blood and the blood volume. This report presents the results of simultaneous studies of the peripheral blood and blood volume in 62 human subjects, 13 with "anemia of neurasthenia," 17 with a decreased plasma volume, 17 with normal and pathologic pregnancy, and 15 with polycythemia vera. METHODS Blood volume and peripheral blood studies were done simultaneously so that the results could be correlated. The hemoglobin concentration was determined by the oxyhemoglobin method of Sheard and Sanford,1using a Coleman Junior