  We present two pieces of evidence supporting the hypothesis that the bright
[OIII] lambda 5007 sources found in Virgo's intracluster space are intracluster
planetary nebulae, rather than [OII] lambda 3727 galaxies at z ~ 0.35 or
Ly-alpha sources at z ~ 3.13. First, we confirm the nature of five
``overluminous'' [OIII] sources that are postulated to lie in front of M87: by
examining the composite spectrum of these objects, we show that the weaker
[OIII] line at lambda 4959 is definitely present at a strength ~ 1/3 that of
[OIII] lambda 5007. The ratio demonstrates that, at most, only one of the five
objects is a background galaxy. We then estimate the surface density of
background emission-line objects by conducting a wide-field (0.13 deg^2) search
at lambda 5019 for faint emission line sources in a ``blank field'' located
well away from any galaxy or cluster. We show that the density of blank field
emission-line sources is significantly lower than the density of sources
detected between the galaxies of Virgo, but in good agreement with the density
of Ly-alpha galaxies found by Hu, Cowie, & McMahon(1998). The implication is
that background galaxies only account for ~20% of the planetary nebula
candidates in Virgo's intracluster fields.
