Standard computer input tools such as a mouse and keyboard do not provide users with a direct and intuitive means of 3-D shape operation. To solve this problem, this paper proposes a new concept of an interface system for 3-D shape manipulation using deformable real objects as input tools. By deforming the tool with bare hands with a tactile feedback, users can deform a 3-D shape modeled and displayed on a computer screen directly and intuitively. By adopting a free-form deformation-oriented approach, the interface enables various types of 3-D shape operations using a rather simple input tool. A prototype of DO-IT (deformable objects as input tools) interface using graphics workstation and a cubical input tool made of foam rubber and strain gauges demonstrates the effectiveness and potential of the concept.