This paper presents an experimental approach for the evaluation of the third body role in dry contacts. The ultrasonic cleaning is applied to remove the third body layer from a contact sample pair, allowing to perform frictional tests with and without the interface layer. The comparison between the observed behaviors evidences a strong influence of the third body on the overall frictional response. The cleaned samples have been used as the substrate to introduce an external third body. The frictional tests pointed out the predominant role of the interface layer, rather than the substrate, on the frictional performances. This procedure allows to test external (eventually artificial) third bodies, investigating the role of different features (morphology, composition) on the frictional response.