Human lymphocytes derived from regional lymph nodes adjacent to the primary gastric cancer were transformed with Epstein-Barr virus (EBV) to establish lymphoblastoid cell lines secreting human antibodies reactive with cell surface antigens expressed on the gastric cancer cells. The EBV transformation technique was applied to lymph node lymphocytes obtained from 4 gastric cancer patients. As a result of mass screening with the radioactive cell binding assay for the production of anti-gastric cancer related antibodies, one culture (TGc-106) among 1,400 microcultures was identified to secrete human antibody specifically reactive with an established human gastric cancer cell line as target (MKN-45). Furthermore, it was demonstrated with the autologous assay system by the histoimmunofluorescence method that cell surface antigens of autologous gastric cancer cells could be clearly defined with human antibody from one culture (TEb-079) out of 470 microcultures established from a gastric cancer patient (GCP-26); there was no reactivity against the surrounding normal cells constructing the gastric wall. The immunoglobulin class of the human antibodies produced both in TGc-106 and TEb-079 was determined from immunodiffusion tests to be IgM.