Preliminary results of a study aimed to the determination of the response function, in direction and energy, of the Gamma-Ray Burst Monitor (GRBM) onboard the BeppoSAX satellite are presented. The study has been carried out by means of Monte Carlo techniques, taking into account on-ground calibration before launch, and the Crab spectrum measured during the flight. The derived response matrix will be used to evaluate properties (e.g., direction, spectrum, fluence) of all Gamma-Ray Bursts detected with the GRBM. ©2000 American Institute of Physics.