This paper describes the background and objectives of the IST project “Organizational Aspects of Human-Machine Coexisting Systems” (HUMACS), that develops and pilot-demonstrates a Participative Simulation environment for Integral (i.e., logistics, technology and human factors) Manufacturing enterprise renewal (PSIM). In the short run, HUMACS/PSIM aims to address the key issues that have to be resolved before a virtual factory can be implemented, and is focused on intelligent manufacturing to assist human creativity and the application of ICT technology. The long-term goal is to show simulated assembly lines in a software environment, to be used in assembly operations to enable a participative improvement process involving specialized staff, management and production personnel. This paper illustrates both the principle and developed prototype of the PSIM tool, including its STEAM procedure.