Full-scale drilling experiments (8½ in. bit size) with a high-pressure fluid jet assisted rotary drilling system were performed in hard to drill crystalline rock. Besides the development of novel drill bits and high-pressure components, an existing drilling simulator test bench was adapted to the requirements of the current tests. A total number of seventeen test runs was conducted to enable a comparison of drilling performance between state of the art drilling technique and the innovative jet assisted system. A significant increase in rate of penetration was achieved with the system. Another relevant aspect was the influence of different drilling and jetting fluids on drilling performance. The interaction of the hydraulic and mechanical rock removing processes was researched in detail and led to further conclusions about the rock destruction process. Basic and sophisticated correlations between drilling parameters were examined in order to define the favourable operational parameters for each bit type. Lessons learned during these experiments will directly be integrated in the preparation of later field tests of the enhanced system.