In a study on dogs, nattokinase was given to dogs orally to see if it could break up clots that were experimentally induced in the dogs.  Bear in mind that the same researchers had shown in previous studies that nattokinase could increase fibrinolytic activity (dissolve the fibrin in blood clots and help wound healing) in the plasma.

The researchers followed the progress of the clots using angiography.

Their results confirmed that orally administered nattokinase could break up the clots in vivo.

Source: http://content.karger.com/ProdukteDB/produkte.asp?doi=10.1159/000205051