Human serum albumin (HSA) dimer, where two molecules of HSA are genetically fused with a linker of 10 amino acid, has superior blood retention property, compared with HSA monomer. Moreover, HSA dimer derivative, s-nitrosated HSA dimer, functions as an enhanced permeability and retention effects enhancer. HSA dimer has gained considerable attention as drug delivery system carrier based on its prominent function. However, for the HSA dimer to be used clinically, the safety profile of the HSA dimer is required in order to exclude any potential toxicity or unwanted effects. Thus, the present study was undertaken to investigate the occurance of tissue damage and serologic changes due to repeated administration of HSA dimer (66.5 mg/kg) to mice every 3 d for 56 d, as part of a basic consideration on safety evaluation. The evaluation on survival, behavior and body weight indicate that HSA dimer has no effect on physical growth and physiological functions. Hematological tests suggest that HSA dimer has no direct influence on hemocytes, such as hemolysis and platelet aggregation. Moreover, plasma clinical chemistry and histological examinations indicate that the HSA dimer has no deleterious effect on liver and renal functions. The results obtained here indicate HSA dimer is safe and should be useful for medical and pharmaceutical applications.