  We use data for 6048 early-type galaxies (ETGs) from Galaxy Zoo 1 that have
been cross-matched with the catalog of the MPA-JHU emission-line measurements
for the Sloan Digital Sky Survey Data Release 7. We measure the metallicity of
these ETGs by excluding various ionization sources, and study other properties
as well. We use the optimal division line of W2-W3 $=$ 2.5 as a diagnostic
tool, and for the first time derive metallicity measurements for 2218 ETGs. We
find that these ETGs actually are closer to H II regions as defined by
Kauffmann et al. in the Baldwin-philips-Terevich diagram, and they display
younger stellar populations. We present a full mass-metallicity relation and
find that most ETGs have lower metallicities than star-forming galaxies (SFGs)
at a given galaxy stellar mass. We use five metallicity calibrators to check
our results. We find that these metallicity indicators (R23, O32, and O3S2)
give consistent results. We suggest that the remaining two metallicity
calibrators, which increase metallicity by N-enrichment, can be used to
calibrate metallicities for SFGs, but not to estimate the metallicities of
ETGs.
