131I-labeled macroaggregates were injected into the femoral artery of rats bearing a Yoshida sarcoma at the lower extremity and of healthy control animals. The radioactivity was measured in tissue samples taken from the tumour and in unaffected muscle tissue. The tumour tissue showed a considerably higher uptake of radioactivity than the not-neoplastic tissue of control animals. The difference was increased when more time elapsed between the injection and the measurement. After 48 hours the ratio was 25:1 in favour of tumour tissue. In contrast to the transplanted Yoshida sarcoma, hepatomas in Wistar rats induced by N-nitroso-morpholin showed a considerably smaller increase of radioactivity than the control group when injected with 131I-MAA. The fixation ratio between tumour and normal liver tissue was 1:5.9. In contrast to autochtone liver tumours rats with transplanted tumours are considered as a suitable model for testing labeled albumin particles like 131I-, 111In- or 99mTc-labelled albumin microspheres developed for angioscintigraphic purposes.