A participatory and integrative approach was applied to improve the productivity and ergonomics of the assembly lines in two factories, producing magnetic stop valves and office furniture, respectively. Main elements of the approach are the active participation of the company and the integration of two disciplines: assembly engineering and ergonomics. The aims were the analysis of bottlenecks and the definition of solutions. To analyse the main bottlenecks, the assembly process scheme was analysed in close cooperation with company representatives, specific observations were performed on the work floor, and the body postures and the forces on the body measured. To evaluate promising solutions a gaming technique (Ergomix) was applied mixing CAD-drawings of work places with real workers. The approach proved to be successful in the determination of bottlenecks in productivity and ergonomics (regarding the supply of components, the accumulation of materials, body postures, pushing and lifting forces), and the definition of promising solutions (e.g. new assembly concepts, height adjustable working tables, new organization of tasks, and new product design). The potential effects were estimated to be significant: e.g. gains in productivity of 15-20% and substantial reductions of time spent in risky body postures.