Histochemical analysis of subcutaneous B16F10 and 4T1 tumor cells 10 days after treatment with hirudin, 10 mg/kg 5 minutes before and for 10 days thereafter. Tumor nodules remained fixed and stained with hematoxylin and eosin. (A) B16 F10 contral nodule. (B) B16 F10 nodule of hirudin-treated animal with central necrosis. (C) KI-67 stain of B16 F10 nodule of untreated control nodule. (D) KI-67 stain of B16 F10 nodule from hirudin-treated animal. (E) 4T1 control nodule. (F) 4T1 nodule following hirudin treatment. Representative of 3 different experiments for each tumor. Original magnification × 250 (A-D) and × 1000 (E, F).