Abstract. A new antibody, anti‐Fy5, has been found in the serum of a Negro Fy(a–b–) person. It reacts with Fy(a+) or Fy(b+) red cells of common Rh type, but not with Fy(a–b–) cells from Negroes, or with Rhnull cells. An example of Caucasian Fy(a–b–) red cells was Fy5 positive. U‐negative or LW‐negative red cells give the reaction expected of their Duffy type. The data suggest that the Fy5 determinant is formed by interaction of Rhesus and Duffy gene products.

A new antibody, anti-Fy5, has been found in the serum of a Negro Fy(a-b-) person. It reacts with Fy(a+) or Fy(b+) red cells of common Rh type, but not with Fy(a-b-) cells from Negroes, or with Rh(null) cells. An example of Caucasian Fy(a-b-) red cells was Fy5 positive. U-negative or LW-negative red cells give the reaction expected of their Duffy type. The data suggest that the Fy5 determinant is formed by interaction of Rhesus and Duffy gene products.