The plasma volume, red cell volume, or both were measured in 170 normal, anemic, or polycythemic subjects. For anemic subjects without a serum protein abnormality or splenomegaly, the relationship between hematocrit and red cell volume was linear and predictable. In patients with a serum monoclonal globulin on electrophoresis, the plasma voluem was significantly increased for the hematocrit in 30%, and the total blood volume was increased in 45%. The frequency of an elevated plasma volume was higher in patients with a markedly increased level of monoclonal protein. Reductions of abnormal proteins with chemotherapy were associated with declines in plasma volume. For a specific concentration, the serum viscosity was highest in patients with IgM proteins and lowest in patients with IgG globulins. Marked elevations in viscosity were noted only in sera with macroglobulinemia or with more than 5 g/dl of IgG or IgA globulins.