  GRB 990123 was a long, complex gamma-ray burst accompanied by an extremely
bright optical flash. We present the collective constraints on the bulk Lorentz
factor for this burst based on estimates from burst kinematics, synchrotron
spectral decay, prompt radio flash observations, and prompt emission pulse
width. Combination of these constraints leads to an average bulk Lorentz factor
for GRB 990123 of Gamma_0=1000 +/- 100 which implies a baryon loading of
M_jet=8 (+17/-2) x 10^-8 Msolar. We find these constraints to be consistent
with the speculation that the optical light is emission from the reverse shock
component of the external shock. In addition, we find the implied value of
M_jet to be in accordance with theoretical estimates: the baryonic loading is
sufficiently small to allow acceleration of the outflow to Gamma > 100.
