An antibody is described, produced by an Fy(a+b-) woman that at first reacted with her own cells, preferentially with Fy(b+) cells but not with Fy(a-b-) cells. Later, pure anti-Fyb specificity occurred. Elution and absorption studies revealed that the antibody recognized a determinant absent from Fy(a-b-) cells but present on Fy(a+b-) and Fy(a-b+) cells, much more strongly, however, on the latter, and that it was an autoantibody.