An immunotoxin containing an epitope of human gastrin conjugated to diphtheria toxin, with antineoplastic activity. The gastrin epitope in this vaccine is chemically identical or similar to the endogenous gastrin-17 (G-17), a 17-amino acid peptide hormone that stimulates secretion of gastric acid by the stomach. Diphtheria toxin inhibits protein synthesis via modifying translation elongation factor 2 (EF-2). Vaccination with this immunotoxin may elicit production of antibodies against gastrinoma cells overexpressing gastrin, in addition to the toxic effects on protein synthesis exerted by the diphtheria toxin moiety.