The main objective of this paper is to improve the assembly lines supply process of a multinational company in the automotive industry. Main goals include the delivery routes normalization taking into account the number of operators needed and their tasks. The process was analyzed to identify the main operational problems and an improvement plan was defined and implemented, by applying the action-research methodology. The main problems identified were ergonomic efforts; lack of efficiency; excessive stock, low ramps capacity; lack of material organization; and inappropriate location of materials. The initial procedure was to create a decision support model implemented through a spread sheet. Later, information regarding the different materials was collected, as well as the capacity of the supply ramps and trolleys. The main problems were identified and improvements were implemented through Lean tools: standard work; pull system and continuous flow; visual management and 5S. It was possible to reduce the supply cycle time and the number of employees needed reflecting in a saving of 108 000 € per year. Also, through the reduction of intermediate stock, it was possible to reduce the number of pallets and trolleys needed corresponding to a total saving of 117 181 €. In sum, it was possible to standardize material supply routes for assembly lines, eliminate waste and improve ergonomics by reducing material handling and workers’ occupation rate.