A new method to constrain the cosmological equation of state is proposed by using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The Chevallier-Polarski-Linder parameterization is adopted for the equation of state in order to find out a realistic approach to achieve the deceleration/acceleration transition phase of dark energy models. We find that GRBs, calibrated by SNeIa, could be good distance indicators capable of discriminating between cosmological models and $Λ$CDM model at high redshift. ; Comment: 5 pages, 5 figures, to appear in Astron. & Astroph