  Recently we have presented a new theoretical analysis of the reverse shock
emission. We use this analysis here to revisit GRB 990123. We find new and
compelling evidences that the optical flash and the radio flare of GRB 990123
resulted from a reverse shock. This suggests that a significant fraction of the
energy of the relativistic ejecta must have been carried by baryons. It also
suggests that the external medium is an ISM and that in this burst the reverse
shock emission dominated at early time over other possible processes. We use
the early optical emission to constrain the physical parameters of the original
ejecta and the microscopic parameters in the emitting reverse shocked region.
