The authors report the results of a phase I clinical study using semiallogeneic cancer vaccines formulated with granulocyte-macrophage colony-stimulating factor (GM-CSF) to treat patients with metastatic adenocarcinomas of the gastrointestinal tract. A specially engineered cell line, FO1-12, was used to generate semiallogeneic hybrids by fusion with patient-derived tumor cells; the hybrids express HLA class I and II haplotypes derived from both parental cells. For treatment, the vaccine was mixed with GM-CSF, irradiated, and injected intradermally into patients at weekly or biweekly intervals. Vaccinations were associated with minimal or no toxicity and showed that semiallogeneic hybrids formulated with GM-CSF can induce a specific antitumor immune response in some patients, as measured by a delayed-type hypersensitivity response to autologous tumor cells. Because of the simplicity, feasibility, and flexibility of this immunotherapeutic approach, semiallogeneic hybrid vaccines have the potential to be used in the treatment of virtually any type of cancer.