Four patients with glioblastoma and one patient with astrocytoma (grade III) were treated with recombinant IFN ( rIFN -alpha A, Ro 22-8181) and the effect of IFN on clinical symptoms, CT findings and side effects of IFN were studied. Neurological symptoms were improved in one patient, stable in one patient and worsened in three patients. In all cases, there was no remarkable change of CT findings but in one case a slight decrease in tumor size was recognized. With regards to IFN side effects general malaise, anorexia, fever, nausea and vomiting were observed clinically, decrease of leukocytes, platelets, erythrocytes, hematocrit, hemoglobin and increase of GOT, GPT, LDH, AL-P were noted in the laboratory findings. These symptoms and change in laboratory findings were not serious, and they recovered spontaneously during or after IFN therapy. In one patient, an increase in IFN neutralizing antibody titer was detected. Since the biological activity of IFN may be diminished and anti-tumor effect cannot be expected in such a patient, the appearance of IFN neutralizing antibody may indicate an important problem in IFN therapy.