To learn the toxicity of xenogeneic tumor-specific immune ribonucleic acid (I-RNA), experimental and clinical studies were carried out. Experimentally, doses of 30 mg/kg, 15 mg/kg, 7.5 mg/kg or 3.75 mg/kg of xenogeneic I-RNA extracted from lymphoid tissues of rabbits sensitized with 105,000 Xg sediments of human gastric carcinoma tissue were injected intraperitoneally into Wistar rats once a day for 30 days. During the period of the study, changes of physiological and biochemical values were examined. In addition, pathological study was made for each organ after the I-RNA administration. There was no death during the study. In the groups of high dose administration, there were poor increase in the body weight, elevation of GOT, GPT and LDH, findings of vacuolar degeneration of hepatic cells, and increase of mesangial matrix and polynuclear glomerulus. Throughout the experiment, the groups given 7.5 mg/kg and 3.75 mg/kg of I-RNA showed no significant difference from control groups. Clinically, 31 cases treated with passive immunotherapy with allogeneic lymphocytes, preincubated with xenogeneic I-RNA, were examined retrospectively for the difference in hematological and biochemical data before and after the therapy. One case showed a transient mild febrile reaction after the therapy. There was no significant difference in hematological and biochemical data. Pathological findings in 3 autopsies after the treatment were reported.