We present spectroscopic observations for six emission-line
objects projected onto the Virgo cluster.
These sources have been selected from narrow band (H_alpha+[NII]) images showing
faint detectable continuum emission and EW>100 Angstrom.
Five of these sources result [OIII]\lambda5007 emitters at z = 0.31, while one
122603+130724 is confirmed to be an HII region belonging to the Virgo cluster.
This point-like source has a recessional velocity of 200 km/s,
and is associated with the giant galaxy VCC873 (NGC 4402).
It has a higher luminosity, star formation rate and metallicity than
the extragalactic HII region recently
discovered near the Virgo galaxy VCC836 by
Gerhard et al. (2002).