Nude mice bearing CX-1 colon tumors were injected with 50 microCi 125I-labeled monoclonal antibody (MAb) B72.3. Radioactivity in tumors was studied with the gamma detecting probe (GDP) on days 1, 3, 7, and 10 after MAb injection. On each day, two mice were sacrificed and sections were examined with autoradiography (ARG), immunoperoxidase methods (IMP), and routine stains. Mean probe counts showed increasing tumor to background ratios and ARG demonstrated a progressive increase in radionuclide in the tumors. The distribution of 125I was primarily around the vascular spaces on day 1, but by day 3 and progressively it appeared in tumor gland lumina and necrotic areas. A regional correlation was shown between radionuclide in vascular spaces and its sequestration in tumor elements.