I shall present a novel method to calculate the stellar mass in AGN host
galaxies. The method is based on the well known properties of the
"Star Forming Sequence" and the recently discovered correlation between
the luminosity of AGN and the star formation rate in their hosts. Given
these relationships, one can calculate M(BH)/M(galaxy) at almost all
redshifts and obtain important information about the co-evolution of AGN
and SF galaxies, the relative duty cycle of SF and AGN activity, and more.