Radiopharmaceuticals are known to interact with the blood components (i.e. the red blood cells, serum proteins etc) but so far, there have been no data regarding their purely mechanical trapping in thrombi. The experiments presented in this communication provide evidence that the technetium-99m labeled albumin macroaggregate ((99m)Tc-MAA), apparently due to its particle size, can be almost quantitatively retained in the in vitro model described. These results can be extrapolated in vivo and offer a plausible explanation for either the "hot spot" artifact, occasionally seen in lung perfusion imaging or for the partial trapping of (99m)Tc-MAA by a thrombus at the tip of a subclavian catheter, as has been recently reported. Control experiments were also run using (99m)Tc-methylene diphosphonate ((99m)Tc-MDP), (99m)Tc(III)-dimercaptosuccinic acid ((99m)Tc(III)-DMSA), (99m)Tc-methoxyisobutyl isonitrile ((99m)Tc-MIBI) and sodium pertechnetate (Na(99m)TcO(4)), in order to study the extent of trapping of these radiopharmaceuticals under identical incubation conditions. (99m)Tc-MDP and (99m)Tc(III)-DMSA exhibited the lowest blood clot uptake (partially non-specific and partially mechanical trapping), while in the case of (99m)Tc-MIBI and Na(99m)TcO(4), besides mechanical and non-specific clot-trapping, transport and retention inside the red blood cells was also observed.