Autoradiography with 125I-labelled antibodies 17-1-A and 11-285-14 (anti-carcinoembryonic antigen) injected singly or together into nude mice carrying two distinct human colorectal cancer xenografts delineates marked changes in distribution and retention of isotope over 72 h, which are relevant to microdosimetry. The antibodies localise independently at low concentrations. Slow accumulation and retention predominantly in membranes of glands and necrotic areas suggest that therapy will succeed best with isotopes whose range, half-life and/or mode of delivery can exploit optimally the greater selectivity of the late retention.