The all-optical amplitude regeneration of both nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) and return-to-zero DPSK (RZ-DPSK) signals at a bit rate of 42.8 Gbit/s is demonstrated experimentally, utilizing saturation of four-wave mixing in a highly nonlinear optical fiber (HNLF). The power transfer curves of the HNLF for both NRZ-DPSK and RZ-DPSK signals are measured. The regenerative performance for both modulation formats are compared at the same average input power of 16 dBm. Experiment results show that the RZ-DPSK signal exhibits better performance of amplitude regeneration.