Sheets of incompletely fixed biological tissue comprising portions for defining valve flaps of a cardiac valve prosthesis are mounted on forming means which can separate in a substantially fluid-tight manner the opposite faces of such portions. These opposite faces are subjected to a fluid pressure difference which produces deformation of the portions towards a conformation substantially identical with the conformation of the valve flaps when mounted in the prosthesis. The biological tissue is finally fixed while the portions defining the valve flaps of the prosthesis are maintained in the said conformation. The sheets of biological tissue are then separated from the forming means to be subsequently mounted in the frame of the prosthesis.