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We present spectrophotometry in the 3600-9700Å 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 T eff 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 & Pagel [O II]+[O III] abundance index and the Vílchez & Pagel η ′ 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 T eff calibration for the relevant temperature range T eff ≤ 38000 K. We use empirical relations between the nebular hardness indices and T eff 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 ≤ Z ⊙ . However unresolved degeneracies between Z and T eff still complicate the interpretation of this result.

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