Immunogens against human extended-progastrin species comprise (A) a mimetic peptide comprised of (i) the amino acid sequence of a progastrin or a N- and/or C-terminal processed species of a progastrin joined to (ii) a 7 amino-acid spacer coupled to (B) an immunogenic carrier. Illustrative of the mimetic peptide/spacer combination are a 21 amino-acid peptide (SEQ ID NO.: 1) and other, related polypeptides (SEQ ID NOs.: 2-5). Pharmaceutical compositions containing such an immunogen display improved immunological properties, including the induction of effective antibody levels shortly after the administration of an initial course of immunogen. Levels of antibody thus elicited stay elevated for several months and readily elevate to higher levels upon subsequent boosting by a single injection of immunogen.