Bovine α-fetoprotein was isolated from bovine fetal serum by an immuno-chemical method. It was comfirmed that this protein was antigenically related to human α-fetoprotein. The levels of α-fetoprotein, albumin, transferrin, immunoglobulin and total proteins in the sera of developing bovine fetuses and calves were assayed by the single radial immuno-diffusion method. The maximum mean value of α-fetoprotein (6.14±0.61 mg/ml within a range of 5-8 mg/ml) was observed in the sera of 4-month fetuses. It decreased to 0.01-0.06 mg/ml at birth. The half-time of disappearance of α-fetoprotein after birth was estimated to be 2.52 days. Serum albumin, serum transferrin and total proteins increased successively with the development of fetuses. IgG was also detected from 4 (11%) of 36 fetal serum samples.