We present spectroscopic observations for six emission-line objects projected onto the Virgo cluster. These sources have been selected from narrow band (Ha + [NII]) images showing faint detectable continuum emission and EW > 100 Å. Five of these sources result [OIII]?5007 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-1, and is associated with the giant galaxy VCC 873 (NGC 4402). It has a higher luminosity, star formation rate and metallicity than the extragalactic HII region recently discovered near the Virgo galaxy VCC 836 by Gerhard et al. (2002).