Microrheological parameters of red blood cells (RBCs) and blood viscosity were studied by different optical and rheometric techniques. Measurements were performed under in vitro condition both on the EDTA-stabilized whole blood samples at various temperatures and on the RBCs in autologous serum without platelets at different concentrations of proaggregant macromolecules (Dextran 150, Dextran 500 and albumin). The nonlinear dependences of the aggregation parameters on the concentration of the proaggregant macromolecules in autologous serum were observed. Rheometry of human whole blood samples demonstrates a power law dependence of the suspension viscosity on temperature at such shear rates that allow the RBC aggregation. We assume that there may be an unaccounted temperature dependent synergetic effect of plasma proteins/macromolecules on RBC aggregation and interaction and, consequently, on the blood viscosity.