A fragment (F1) of human serum albumin of molecular weight 6600 and carrying one of the determinant groups of HSA has been injected into rabbits and demonstrated to induce the formation of antibodies. The rabbit immune sera were shown to agglutinate red blood cells sensitized with F1 as well as with HSA. Anti-F1 antibodies were isolated by adsorption on a column of insoluble F1-benzylcellulose conjugate. They were shown to consist solely of IgG. They precipitated slightly with F1 but not with HSA. They agglutinated red blood cells sensitized with F1 or HSA. Both F1 and HSA inhibited these agglutination reactions.