The new technology of laser-driven ion acceleration (LDA) has shown the potential for driving highly brilliant particle beams. Laser-driven ion acceleration differs from conventional proton sources by its ultra-high dose rate, whose radiobiological impact should be investigated thoroughly before adopting current clinical dose concepts. The growth of human FaDu tumors transplanted onto the hind leg of nude mice was measured sonographically. Tumors were irradiated with 20 Gy of 23 MeV protons at pulsed mode with single pulses of 1 ns duration or continuous mode (∼100 ms) in comparison to controls and to a dose-response curve for 6 MV photons. Tumor growth delay and the relative biological effectiveness (RBE) were calculated for all irradiation modes. The mean target dose reconstructed from Gafchromic films was 17.4 ± 0.8 Gy for the pulsed and 19.7 ± 1.1 Gy for the continuous irradiation mode. The mean tumor growth delay was 34 ± 6 days for pulsed, 35 ± 6 days for continuous protons, and 31 ± 7 days for photons 20 ± 1.2 Gy, resulting in RBEs of 1.22 ± 0.19 for pulsed and 1.10 ± 0.18 for continuous protons, respectively. In summary, protons were found to be significantly more effective in reducing the tumor volume than photons (P < 0.05). Together with the results of previous in vitro experiments, the in vivo data reveal no evidence for a substantially different radiobiology that is associated with the ultra-high dose rate of protons that might be generated from advanced laser technology in the future.