A general-purpose servohydraulic machine used for testing materials or manufactured components is a high-performance mechatronic system : wide bandwidths and fast sampling are requried for typical tests. Accurate results need good consistent control performance, but changes of stiffness of the test specimen markedly affect closed-loop natural frequency and damping. Moreover, a new specimen type often necessitates controller returning-a procedure that is unfamiliar to most materials engineers. An adaptive controller is required to maintain performance irrespective of the specimen type or of changes of its stiffness during the cycle. This paper describes a successful adaptive ’grey-box’ controller that has been used on over 300 different testing machines. A simplified machine model, based on physical principles, has parameters dependent on the specimen stiffness, and a test procedure using an arbitrary specimen provides initial model and controller settings. Two single-parameter recursive estimators track changing stiffnesses, and the derived values are used to retune PI parameters according to a simple algorithm. This adaptive controller maintains bandwidth even if the specimen stiffness varies during a single test cycle, unlike black-box adaptive controllers, for which lack of persistency of excitation precludes the required parameter tracking.