Endothelin converting enzymes of mammalian cell origin or mammalian blood origin are disclosed, which have the activity of converting big endothelin to endothelin and a hydrated density of up to about 0.94 g/ml, about 0.94 g/ml to about 1.006 g/ml, about 1.006 g/ml to about 1.063 g/ml and about 1.063 g/ml to about 1.210 g/ml. A process for preparing the enzymes from mammalian cells and mammalian blood by centrifugation or gel filtration process is also disclosed.