  We reconsider the use of the equivalent width of CIV 1549, EW(CIV), as an
indicator of the oxygen abundance in star-forming galaxies, as proposed by
Heckman et al. for nearby starbursts. We refine the local calibration of
EW(CIV) vs. log (O/H) by using a restricted wavelength window which minimises
blending with interstellar absorption lines. When applied to the stellar
component only of the complex CIV 1549 features in two high redshift galaxies
with good quality spectra, MS1512-cB58 (z = 2.7268) and Q1307-BM1163 (z =
1.4105), the local calibration gives values of the oxygen abundance which are
in good agreement with other metallicity determinations based on nebular
emission and interstellar absorption lines. Our main conclusion is that for
this method to give reliable results at high redshifts, it should only be used
on data of sufficiently high spectral resolution (R > 1000) for stellar and
interstellar CIV components to be clearly separated. Oxygen abundances will be
systematically overestimated if the local calibration is applied to spectra of
high-z galaxies obtained with the low resolving powers (R ~ 200-300) of many
current wide field surveys. It will also be necessary to understand better the
causes of the scatter in the local relation, before we can be confident of
inferences from it at high z.
