  We point out that the already existing literature on relativistic
collisionless MHD shocks show that the parameter sigma= upstream proper
magnetic energy density/upstream rest mass energy density, plays an important
role in determining the structure and accelerating properties of such shocks.
By adopting a value of sigma= 0.002 which corresponds to the relativistic shock
associated with the Crab nebula, and by using appropriate relativistic shock
jump conditions, we obtain here a generous upper-limit on the value of (proper)
the magnetic field, B ~ 1.5 10^{-3} eta n^{1/2} G, for gamma ray burst (GRB)
blast wave. Here, eta= E/Mc^2, where E is the energy and M is the mass of the
baryons entrained in the original fireball (FB), and n is the proper number
density of the ambient medium. Further, we point out that, in realistic cases,
the actual value B could be as low as 5 10^{-6} eta n^{1/2} G. for realistic
cases.
