The white blood cell count in the peripheral blood decreased to 57% of the control in ddY mice after intraperitoneal administration of 3-azido-3-deoxythymidine (AZT, 500 mg/kg/day), mitomycin C (MMC, 1 mg/kg/day), or 5-fluorouracil (5-FU, 50 mg/kg/day) for 7 days or general gamma-irradiation at 35 rad. However, this reduction was significantly prevented by administering L-carnosine (CAR) or beta-alanine (beta-ALA) simultaneously or subcutaneously for 7 days from the day after irradiation, suggesting an anti-leukopenic effect of CAR. When Wistar rats were administered phenylhydrazine (PHZ, 40 mg/kg) twice 1 and 3 days before evaluation, the red blood cell count was reduced to 55% of the control. However, the reduction was to 69% in the group treated with CAR for 8 days from 9 days prior to evaluation. The hematocrit and hemoglobin level were also increased by the administration of CAR, suggesting a protective effect of the agent against hemolytic anemia. Since membrane stabilization is considered to be the mechanism of this effect lysosome-rich fraction isolated from the liver of Wistar rats were incubated in 0.2 M sucrose with CAR, and the acid phosphatase activity released into the incubation medium was measured. CAR was found to have a membrane-stabilizing effect, which reached a plateau at a final concentration of 2.5 mM. This membrane stabilizing effect was not observed with beta-ALA or L-histidine (HIS) alone at a final concentration of 5 mM, and the release of the enzyme was only slightly inhibited by HIS + beta-ALA. Therefore, CAR molecules are considered to be needed for membrane stabilization.