Monovalent targeting of FcRI prevents and arrests subcutaneous tumor development by FcαRI RBL transfectants in nude mice. Nude mice (6/group) were injected subcutaneously with WT FcαRI RBL transfectants. In the prevention model (A,C), animals were treated intraperitoneally with 50 μg anti-FcαRI (A77) Fab, control (320) Fab, or PBS on days 0, 3, 6, 9, 12, 15, 18, and 21. In treatment model (B,D), mice were treated intraperitoneally with 50 μg anti-FcαRI (A77) Fab, control (320) Fab, or PBS on day 12 when tumors had started to develop followed by injections on days, 15, 18, and 21. (A-B) Tumor size was evaluated on day 25 and is represented as the mean ± SD (n = 6). (C-D) Survival of mice treated with PBS, 320 Fab, or A77 Fab. (E) TUNEL analysis of RBL tumor cells after treatment with control (320) Fab or anti-FcαRI (A77) Fab. Images show PAS staining of formalin-fixed tumor sections. Apoptotic cells were visualized by TUNEL analysis of cryosections as described in “Materials and methods (10×/0.30 NA objective lens).