Résumé : We introduce a nonparametric methodology for analyzing production technologies characterized by weakly separable structures involving intermediate inputs. Weakly separable production functions allow for modeling production stages where raw inputs aggregate into intermediate inputs before interacting with other factors of production. We establish nonparametric testable conditions for cost minimizing behavior that accommodate production with intermediate inputs. These conditions offer practical tools for empirical analysis of separable production processes. We show that they provide a useful basis for measuring productive efficiency and for identifying shadow prices of intermediate inputs. Additionally, we discuss potential extensions to handle jointly used raw inputs, multiple final outputs, and scenarios with limited price and quantity information.