Platelet-free plasma of human blood (sodium citrate and EDTA as an anticoagulant) and serum were stored at 4°C, room temperature (25°C) and at 37°C for 24 hours. RBC aggregation decreased after incubation of plasma and serum at 37°C for 4 hours. The RBC shape was changed at the same time: discocytes transformed to echinocytes. Storage of plasma and serum at 4°C and room temperature did not lead to significant alterations of RBC aggregation. The RBC shape did not change in influence of such plasma and serum. The most considerable decrease of RBC aggregation and change of their shapes were observed in the plasma and serum incubated at 37°C for 24 hours. Dilution of incubated plasma by fresh plasma led to consistent restoration of erythrocyte shape and their aggregation.