  We present spectrophotometry in the 3600-9700 A region for a sample of 39 H
II regions in the Galaxy and Magellanic Clouds, for which independent
information is available on the spectral types and effective temperatures of
the ionizing stars. The spectra have been used to evaluate nebular diagnostics
of stellar temperature, metal abundance, and ionization parameter, and compare
the observed behavior of the line indices with predictions of nebular
photoionization models. We observe a strong degeneracy between forbidden-line
sequences produced by changes in stellar Teff and metal abundance, which
severely complicates the application of many forbidden-line diagnostics to
extragalactic H II regions. Our data confirm however that the Edmunds and Pagel
[O II]+[O III] abundance index and the Vilchez and Pagel `eta' index provide
more robust diagnostics of metal abundance and stellar effective temperature,
respectively. A comparison of the fractional helium ionization of the H II
regions with stellar temperature confirms the reliability of the spectral type
vs Teff calibration for the relevant temperature range Teff < 38000 K. We use
empirical relations between the nebular hardness indices and Teff to
reinvestigate the case for systematic variations in the stellar effective
temperatures and the upper IMFs of massive stars in extragalactic H II regions.
The data are consistent with a significant softening of the ionizing spectra
(consistent with cooler stellar temperatures) with increasing metal abundance,
especially for Z less than solar. However unresolved degeneracies between Z and
Teff still complicate the interpretation of this result.
