  Long exposure, long-slit spectra have been obtained in the UV/optical bands
for two galaxies at z=2.96 and z=3.32 to investigate the fraction of ionizing
UV photons escaping from high redshifts galaxies. The two targets are among the
brightest galaxies discovered by Steidel and collaborators and they have
different properties in terms of Lyman-alpha emission and dust reddening. No
significant Lyman continuum emission has been detected. The noise level in the
spectra implies an upper limit of f_{rel,esc}\equiv 3 f(900)/f(1500)< 16% for
the relative escape fraction of ionizing photons, after correction for
absorption by the intervening intergalactic medium. This upper limit is 4 times
lower than the previous detection derived from a composite spectrum of 29 Lyman
break galaxies at z 3.4. If this value is typical of the escape fraction of the
z 3 galaxies, and is added to the expected contribution of the QSO population,
the derived UV background is in good agreement with the one derived by the
proximity effect.
