It has been demonstrated that placenta extract of rats contains up to 14 antigens. Moreover, 11 of them are interorgan proteins of wide and limited specificity, two antigens (alpha 1- and alpha 2-globulins) are attributed to acute-phase proteins typical for pregnancy. beta 1-Globulin is a specific protein of rat placenta. The content of these antigens in blood serum increases with pregnancy and reaches a maximum toward the delivery; 3-4 days after delivery beta 1-globulin disappears completely from maternal blood, whereas the concentration of acute-phase proteins drops to the initial level.