  We present the first detailed spectroscopic study of field galaxies up to
z=0.3 from the I-magnitude limited CFRS sample. In this complete sample, we
find that about half of the objects are blue emission-line galaxies. Using line
ratio diagram and photoionization models, we show that in roughly half of these
emission-line galaxies, ionization by hot stars cannot account for the observed
line ratios. Their spectra have properties intermediate between Seyfert 2
galaxies and low-ionization nuclear emission-line regions (LINERs). Their
number density (about 20%) relative to the total number of galaxies up to
z=0.3, is much larger than the number density (2%) of active galactic nuclei
(AGN) in the local Universe. Thus, the observed excess of blue galaxies in
counts down to B=24 cannot be mainly due to starburst galaxies, as was
previously thought. This result can shed a new light on the nature of the
overabundant population of blue galaxies at low redshifts.
