This paper studies testing in an object-oriented (OO) development process. It is based on the production cell case study. A control program for the production cell has been developed using both the Fusion method, for OO analysis and design, and the formal notation CO-OPN, as an intermediate between the OO design and an implementation in Ada 95. The paper describes the application of a statistical testing method, developed at LAAS, where test inputs are derived from the OO analysis documents, and the application of a formal testing method, developed at EPFL, where test inputs are derived from CO-OPN descriptions. Then various problems which have appeared during the case study are reported. They mainly concern controllability and observability issues and caused some iteration and backtrack on OO analysis and design. The paper concludes by sketching a notion of OO design for testability.