The authors have investigated whether specific pathological changes and antibodies against interleukin-2 (IL-2) are induced after intracerebral administration of recombinant IL-2 (rIL-2). In addition, IL-2 receptor (IL-2R) expression was checked on the cell surface of normal brain tissues before and after the intracerebral infusion. Reconstituted rIL-2 (specific activity 1.2 x 10(7) U/mg protein) was injected into the right cerebral hemisphere of normal adult C57BL/6 mice in three different dose groups, each receiving single or multiple infusions of 8, 32, or 80 U. In sham control experiments, mouse albumin purified by gel filtration and ion exchange chromatography and adjusted to the same concentration of protein as rIL-2 was injected into mice at various doses. Anti-IL-2 antibodies were measured by an enzyme-linked immunosorbent assay concurrently with assessment of IL-2 activity in serum. The IL-2R expression was determined by using immunofluorescence techniques with monoclonal antibodies against mouse IL-2R. Since histological alteration after rIL-2 injection did not differ from that in the sham control preparations, it seems that there is no direct toxic action of rIL-2 on normal brain tissues. Interleukin-2 antibodies were produced at low levels only in mice injected repeatedly at the maximum dose, and levels were insignificant in other groups. Serum levels of IL-2 activity remained low. The IL-2R expression within the brain was not enhanced within 8 weeks following the intracerebral administration of rIL-2, suggesting that direct intracerebral infusion of rIL-2 may be safely used in the immunotherapy of brain tumors.