Hemoglobin vesicle (HbV), a liposomal oxygen carrier containing human hemoglobin, was intravenously infused into rats. After the infusion of saline, the HbV or empty vesicle (EV), numbers of red cells, leukocytes and platelets in peripheral blood were unchanged during the observation period of one week in addition to each time point among three groups. However, the lymphocyte ratio transiently decreased and the granulocyte ratio increased in the HbV and EV groups at 6 h after the infusion. Those changes returned to the initial value one day after the infusion and those were maintained for the subsequent observation period. No dramatic change was seen in the ratio of CD4(+)/CD8(+) T cells. A transient decrease of the complement titer was observed three days after the infusion of HbV and EV, although the consumption of complement titer was not detected in rat serum by mixing HbV or EV in vitro, indicating that the transient decrease of complement titer in vivo was not due to the consumption of complement due to the interaction with HbV or EV. Multiple infusions of HbV caused the decrease of complement titer only after the first infusion and no allergic reaction was observed. No anaphylactic shock was observed in rats administered with EV several times, while ovalbumin (OVA) sensitized rats died with symptoms of respiratory distress after the second OVA administration. These results indicate that HbV could be administered without serious clinical symptoms or adverse reactions.