1. Serum proteins of brown hare (Lepus europaeus Pallas, 1778) were studied by the use of 1D PAGE, 2D agarose-PAGE, immunoblotting, inhibitions of trypsin and chymotrypsin, and specific staining for esterase. 2. Some serum proteins were identified, and easily interpretable polymorphisms were found in transferrin alpha 1B glycoprotein, protease inhibitors ATC2, ATC3 and AT1, esterase ES1 and in an unidentified postalbumin PO. 3. On the basis of family studies the evidence was obtained that the variants observed in these polymorphic proteins are under genetic control by codominant alleles of autosomal loci.