Three pregnancy-associated proteins were demonstrated immunologically in the sera of pregnant rats. Two of these proteins were identified as alpha-fetoprotein and alpha-macrofetoprotein. The third high molecular weight (382,000 daltons) pregnancy-associated alpha 2-glycoprotein (alpha 2-PAG) was detected in the sera of pregnant, estrogen-treated and breast cancer-bearing female rats, and gave immunologic reactions of identity. The serum concentration of this alpha 2-glycoprotein increased rapidly during pregnancy and reached a maximum during the third trimester of gestation, then decreased to trace levels within 3 days postpartum. Administration of estrogen to normal adult female rats resulted in markedly elevated serum levels of this alpha 2-glycoprotein. These animal data indicate that (1) pregnancy-associated alpha 2-glycoprotein detected in pregnant, breast-cancer bearing and estrogen-treated rats in analogous to its human counterpart, and (2) this animal model system is suitable to study sites of synthesis, and the biologic function of this pregnancy-associated protein.