The invention relates among others to antibodies comprising a first antigen-binding site that binds Erb B-2 and a second antigen-binding site that binds Erb B-3. The antibodies can typically reduce a ligand-induced receptor function of Erb B-3 on a Erb B-2 and Erb B-3 positive cell. Also described are method for the treatment and use of the antibodies in imaging and in the treatment of subjects having an Erb B-2, Erb B-3 or Erb B-2/3 positive tumor.