Visualization of vessels in human colon carcinoma. Immunochemistry sections. The tumor sections were incubated with monoclonal CD31 antibody (PECAM) using the immunoperoxidase technique. The vessel walls (arrows) stained golden-brown. (A) Human colon carcinoma treated with murine IgG. (B) Human colon carcinoma treated with mAb C11C1. There were fewer vessels apparent in the C11C1-treated group. Bars represent 100 μm.