Histological and immunohistochemical analyses of tumor-bearing mice after fusion protein therapy. Five days after intravenous inoculation with 5 × 10⁴ NXS2 cells, mice with established liver metastases were treated by daily (×5) intravenous injections of either 10 μg ch14.18–IL-2 fusion protein, an equivalent mixture of 10 μg ch14.18 and 30,000 IU rhIL-2, or PBS (pH 7.4). Twenty-four hours after completion of treatment, paraffin-embedded sections of livers from either PBS (A), IL-2/antibody mixture (B), and fusion protein (C) treated animals were stained with hematoxylin/eosin. Tumor foci or infiltrates are depicted at a magnification of 1,000×. Arrows delineate tumor foci (A and B) or inflammatory cells (C). Frozen liver sections of mice treated with fusion protein were stained with monoclonal anti-leukocyte CD45 antibody (D), anti-asialo GM1 antiserum (E), and monoclonal anti-CD8 antibody (F). Red cells indicate positive staining for each marker.