  We present the results of a detailed study of the interstellar medium of MS
1512-cB58, an L* Lyman break galaxy at z = 2.7276, based on new spectral
observations obtained with the Echelle Spectrograph and Imager on the Keck II
telescope at 58 km/s resolution. We focus in particular on the chemical
abundances and kinematics of the interstellar gas and our main findings are as
follows. Even at this relatively early epoch, the ISM of this galaxy is already
highly enriched in elements released by Type II supernovae; the abundances of
O, Mg, Si, P, and S are all about 2/5 of their solar values. In contrast, N and
the Fe-peak elements Mn, Fe, and Ni are underabundant by a factor of about 3.
Based on current ideas of stellar nucleosynthesis, these results can be
understood if most of the metal enrichment in cB58 has taken place within the
last 300 million years, the timescale for the release of N from intermediate
mass stars. cB58 appears to be an example of a galaxy in the process of
converting its gas into stars on a few dynamical timescales; quite possibly we
are witnessing the formation of a galactic bulge or an elliptical galaxy. The
energetic star formation activity has stirred the interstellar medium to high
velocities of up to 1000 km/s. The net effect is a bulk outflow of the ISM at a
speed of 255 km/s and at a rate which exceeds the star formation rate. It is
unclear whether this gas will be lost or retained by the galaxy. We point out
that the chemical and kinematic properties of cB58 are markedly different from
those of most damped Lyman alpha systems at the same redshift.
