  Afterglow observations are commonly used to determine the parameters of GRB
explosions, the energy E, surrounding density n, post-shock magnetic field
equipartition fraction \epsilon_B and electron equipartition fraction
\epsilon_e, under the frequently made assumption that the efficiency of
electron "injection" into relativistic shock acceleration is high, i.e. that
the fraction f of electrons which undergo acceleration is f~1. We show that the
value of f can not be determined by current observations, since currently
testable model predictions for a parameter choice
{E'=E/f,n'=n/f,\epsilon'_B=f\epsilon_B,\epsilon'_e=f\epsilon_e} are independent
of the value of f for m_e/m_p< f< 1. Current observations imply that the
efficiency f is similar for highly relativistic and for sub relativistic
shocks, and plausibly suggest that f~1, quite unlike the situation in the Crab
Nebula. However, values m_e/m_p< f<< 1 can not be ruled out, implying a factor
m_e/m_p uncertainty in determination of model parameters. We show that early,
<10 hr, radio afterglow observations, which will be far more accessible in the
SWIFT era, may provide constraints on f. Such observations will therefore
provide a powerful diagnostic of GRB explosions and of the physics of particle
acceleration in collisionless shocks.
