The biodistribution of iodine-labelled alpha-fetoprotein ( I-AFP) in experimental mammary tumours was studied. C3H mice with subcutaneously transplanted mammary adenocarcinoma and Sprague-Dawley rats treated with -methyl- -nitrosourea for mammary adenoma induction were used as animal models. The accumulation of labelled I-AFP in mouse mammary adenocarcinoma was significantly higher than that in rat mammary adenoma. The tumour/muscle radioactivity ratios increased with time and, 48 h after intravenous injection, were estimated as 23.4 and 6.7, respectively. For experiments, extracts from both mammary tumours were prepared. The extracts were subjected to sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), transferred to polyvinylidene difluoride (PVDF) membranes and incubated with I-AFP. A single major AFP-binding protein with a molecular weight of about 30 kDa was detected in both extracts. The amount of AFP-binding protein was clearly higher for adenocarcinoma than for adenoma. In the presence of cross-linking reagent, I-AFP formed a complex (about 100 kDa) with adenocarcinoma proteins.