After bone damage of the fetal rat femurs induced by administrating cyclophosphamide(CP),(1/8 LD50) to the pregnant rat on 13th day of gestation, the effects of serum and ascorbic acid on the repair process of the bone during organ culture were studied, histologically and scanning electron microscopically. CP-damaged fetal femurs harvested at 20 days of gestation were cultured fro 2, 5 and 7 days in the waymouth media(WM) with or without fetal bovine serum(FBS) and ascorbic acid, and were observed with light microscope and JSM-35C scanning electron microscope. The results were as follows:1. CP-damaged bone tissue cultured in WM with 10% FBS showed relatively enhanced activities in the differentiation of chondrocytes and ossificstion as compared to that cultured in WM. 2. CP-damaged bone tissue cultured in WM with 10% FBS and 100µg/ml ascorbic acid, showed increase in the length of the bone marrow cavity, and active formation of new osteoid and collagen bundles. 3. The bone tissues cultured in WM with 10% FBS and 400µg/ml ascorbic acid revealed active deposition of bone matrix, thickening of periosteum and marked elongation of the bone marrow cavity. 4. Bone trabeculae of CP-damaged femurs cultured for 2 days in WM showed poor cell proliferation and insignificant bone matix formation. 5. The number of new cells and the amount of the collagen fibrils increased on the bone trabeculae of the bone cultured in WM with 10% FBS as compared to that cultured in WM and this increase was enhanced as the culture time progressed. 6. A remarkable increase was noted in the number of cells and collagen fibrils in the bone tissues cultured in WM with 10% FBS and ascorbic acid than in those cultured in WM with 10% FBS. 7. The number of the spherules formed by cellular component with collagen fibrils is more numerous than that formed by calcospherites associated with collagen fibrils.