Summary Electrophoresis patterns show rapid changes in the plasma of developing chick and pig embryos. In the ultra-centrifuge only a single component is apparent in the unfractionated plasma. After electrophoretic fractionation of the plasma or sera from older fetuses, traces of other components are revealed. Diffusion and sedimentation constants of 11-day chick embryo serum would indicate molecular weight of more than 200,000. The diffusion rate of the plasma from older chick fetuses was greater. Pig fetus plasma had much larger diffusion constant, indicating molecular weight of approximately 60,000 for the bulk of the plasma protein from 180 and 250 mm fetuses. Thirteen-day-old chick serum protein contained 6.0% N and 20 to 30% carbohydrate, whereas 180 mm pig embryo serum protein contained 12.5% N and 15 to 20% carbohydrate.