A machine (1) for forming a negative cast (2) of an end portion (3) of an amputated limb (5) has an outer rigid tubular body (7) an elastically deformable tubular membrane (13) housed within the rigid tubular body (7) and having its opposite annular end portions (13a) (13b) fluid- tightly connected to the rigid tubular body (7), to define a through- duct (14) through which the limb (5) passes and, with the rigid tubular body (7), a fluid-tight elongated annular chamber (16) with variable volume surrounding the limb (5) a reference membrane (21) for positioning of the limb (5) within the duct (14) being provided at the outlet of the duct (14).
