purpose/objective The AD-4/ACE collaboration has recently performed experiments to directly measure the RBE of antiprotons. Antiprotons have very similar stopping power compared to protons, but when they come to rest, antiprotons will annihilate on a target nucleus and thereby release almost 2 GeV of energy. About 30 MeV of this energy is deposited in the vicinity of the Bragg-peak, thereby significantly enhancing it. It is furthermore expected that this additional energy is deposited by radiation which carries a high-LET component. This will have a significant influence on the radiobiological properties of the antiproton beam. RBE determination of the antiproton beam is complicated by dosimetry of the mixed particle environment from antiproton annihilation, the pulsed extraction of the antiproton beam, and its relatively low dose rate. Only recently, we have successfully benchmarked a Monte Carlo particle transport code against dosimetry experiments. This finally enables us to determine the relative biological efficiency of a beam of antiprotons. materials/methods We have performed dosimetry experiments and investigated the radiobiological properties using an antiproton beam line at the European nuclear research facility CERN. A beam of 126 MeV antiprotons, corresponding to about 12 cm range in water, was spread out to a SOBP with a width of 1 cm. Dosimetry experiments were carried out with ionization chambers, alanine pellets and radiochromic film, and the results were used for benchmarking Monte Carlo calculations. Radiobiological experiments were done with Chinese V79 WNRE hamster cells, where we measure RBE for 10 % clonogenic survival. The radiobiological experiments were repeated with protons and carbon ions at TRIUMF and GSI, respectively, for comparison. results Preliminary results indicate that the RBE of antiprotons in the plateau region is only slightly increased compared to protons. Monte Carlo calulations of the LET-spectrum of the particles in the plateau region support the idea of an RBE of about 1.2. In the spread out Bragg-peak we measure an RBE of about 1.9, which is only slightly less than that of carbon ions, where we find an RBE of 1.4 in the plateau and 2.2 in the SOBP. However, the antiproton results are still preliminary since the Co-60 validation experiments are still to commence in near future. discussions/conclusion We believe that the increased RBE of antiprotons in the plateau region is due to the presence of annihilation fragments from inflight annihilation. Even though the RBE in the peak region is slightly reduced compared to that of carbon ions, the ratio of peak to plateau is nearly identical. A striking difference between antiprotons and carbon ions can be seen in the survival data for the transition region between plateau and peak region. What consequences this may have for the potential outcome of antiproton treatment is still to be investigated by performing antiproton treatment planning.