The viability and surface morphology of human erythrocytes upon interaction with oligochitosan (OCH), having a molecular weight (MW) of 6.2-15.4 kDa and a degree of acetylation (DA) of 1-2%, and interaction with N-reacetylated OCH (ROCH) with a 6.4-14.3 kDa MW and 24-30% DA were studied in isotonic saline phosphate buffer with pH 7.4. It was shown that the use of OCH caused high hemolysis and irreversible transformation of the erythrocytes. Thus, OCH having a 6.2 kDa MW and 1% DA, used at a 0.01% concentration, induced high hemolysis of erythrocytes, and their viability did not exceed the maximal value of 60%. Among the nonhemolyzed erythrocytes, about 20% reversibly transformed erythrocytes and about 20% irreversibly transformed erythrocytes were observed in comparison with the control experiments. For the first time, it was shown that ROCHs had a much lower impact on the cells. Thus, about 82% of the erythrocytes had a discoid form, while 12% and ∼6% of the cells underwent reversible and irreversible transformations, respectively, in the presence of ROCH (MW 6.4, DA 24%), used at a 0.01% concentration. It was observed that an increase in the MW and concentration of chitosan derivatives led to a decrease in the cell viability. It was supposed that the complexation of chitosan derivatives with phosphate counterions in the buffer might reduce the impact of chitosan derivatives on the viability and surface morphology of erythrocytes due to a reduction in the average zeta-potential of chitosan derivative/phosphate complexes from positive to negative values. These results supported the suggestion that reacetylation and reduction of the overall charge of chitosan molecules could improve the compatibility of chitosan derivatives with erythrocytes. This finding opens an opportunity for the construction of chitosan derivatives and their complexes that are compatible with other blood forming elements.