It was studied whether antibody‐mediated suppression of the immune response was determinant specific. Fluorescein isothiocyanate (FITC)‐conjugated sheep red cells were used as the immunogen and antibodies were raised both against the carrier sheep red cells and the hapten FITC. When these antibodies were injected 1‐3 h after the immunogen they only suppressed the immune response to the corresponding determinant. Anti‐carrier antibodies usually enhanced the response to the hapten. Therefore, antibody‐mediated suppression of the immune response is determinant specific and cannot be mediated in vivo to a detectable extent by the Fc part of the antibodies.