  We present new spectroscopic data of the BL Lac RGB 2243+203, and its
surroundings, obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted
in the Gran Telescopio Canarias (GTC). The spectra of neither the BL Lac nor
its host galaxy show any spectral feature, thus hindering direct determination
of its redshift. The spectroscopic redshift distribution of objects in the MOS
field of view shows four galaxies with redshift between 0.5258 and 0.5288. We
make use of a statistical analysis to test the possibility that the targeted BL
Lac may be a member of that group. By using the spectroscopic redshifts
obtained with our GTC observations, we found that this probability is between
86% and 93%.
