Antibody localization was analyzed following intraperitoneal (i.p.) or intravenous (i.v.) injection of the 125I-labelled anti-CEA-MAb I-38S1 in 44 nude rats, in order to evaluate the influence of tumour implantation site and the route of MAb administration. The animals were xenografted with a human colonic cancer (LS 174 T), either in the form of hepatic metastases, subcutaneous (s.c.) tumours or both. Tissue measurements, 4 days after MAb injection, showed better uptake for hepatic than for s.c. tumours, irrespective of the route of antibody administration. Antibody accumulation per g liver metastases was not size dependent for noduli weighing between 4 and 1,110 mg. MAb excretion evaluated in 20 animals and blood activity studied in 11 rats were equivalent 24-96 h following i.p. and i.v. injection. Dissimilar autoradiographic patterns were seen in hepatic metastases with predominantly peripherally located clusters following i.p. and more homogeneously distributed grains after i.v. MAb administration. The results indicate that tumour implantation site has a quantitative, and the route of administration at least a qualitative impact on the tumour accretion of anti-CEA MAb I-38S1 in the present xenograft model.