Immunization of CBA and C57 mice with a branched, multichain synthetic polypeptide, poly (tyr,glu)-poly DL-ala--poly lys, ((T,G)-A--L), in Freund's complete adjuvant results in a tenfold or more difference in the antigen-binding capacity of sera from the two strains, although they respond equally to bovine serum albumin. Immunization of CBA x C57 F₁, F₁ x CBA, and F₁ x C57 mice reveals definite genetic control of the response to (T,G)-A--L, which appears to be due to a single major genetic factor, with perhaps one or more modifying factors. Immunization of CBA and C57 mice with (H,G)-A--L, a synthetic polypeptide in which histidine replaces tyrosine, gives the opposite result, CBA's respond and C57's do not. From this, it appears that the genetic control of the response to (T,G)-A--L is specific for the antigenic determinant. The implications of these results are discussed.