Although small cell lung cancer (SCLC) is highly responsive to chemotherapy, relapses are common, and most patients die within 2 years of diagnosis. After initial therapy, standard treatment is observation alone. We have been investigating immunization against selected gangliosides as adjuvant therapy directed against residual and presumably resistant disease persisting after chemotherapy and irradiation. Previously, we reported that the presence of anti-GM2 ganglioside antibodies is associated with a prolonged disease-free survival in patients with melanoma, and that SCLC patients immunized with BEC2, an anti-idiotypic monoclonal antibody that mimics the ganglioside GD3, had a prolonged survival compared with historical controls. In the present trial, fucosyl-alpha1-2Galbeta1-3GalNAcbeta1-4(NeuAcalpha2-3) Galbeta1-4Glcbeta1-1Cer (Fuc-GM1), a ganglioside expressed on the SCLC cell surface, was selected as a target for active immunotherapy. Fuc-GM1 is present on most SCLCs but on few normal tissues. SCLC patients achieving a major response to initial therapy were vaccinated s.c. on weeks 1, 2, 3, 4, 8, and 16 with Fuc-GM1 (30 microg) conjugated to the carrier protein keyhole limpet hemocyanin and mixed with the adjuvant QS-21. Ten patients received at least five vaccinations and are evaluable for response. All patients demonstrated a serological response, with induction of both IgM and IgG antibodies against Fuc-GM1, despite prior treatment with chemotherapy with or without radiation. Posttreatment flow cytometry demonstrated binding of antibodies from patients' sera to tumor cells expressing Fuc-GM1. In the majority of cases, sera were also capable of complement-mediated cytotoxicity. Mild transient erythema and induration at injection sites were the only consistent toxicities. The Fuc-GM1-KLH + QS-21 vaccine is safe and immunogenic in patients with SCLC. Continued study of this and other ganglioside vaccines is ongoing.