A comparative study of the toxicity and hemocompatibility of chitosan and its derivatives with different acetylation degrees, molecular masses, charges, and hydrophobicity has been performed. It has been shown that only positively charged chitosan derivatives activate platelets and suppress cell proliferation, regardless of the acetylation degree, molecular mass, and hydrophobicity. Chitosan quaternization decreases toxicity at a low degree of substitution and abruptly increases it at a high one. Negatively charged chitosan derivatives were nontoxic and compatible with blood components. It was concluded that the toxicity of chitosan and its derivatives is defined by their charge and solubility at a neutral pH.