We propose a deep survey of galaxies at 4.5 microns
 This wavelength is near the minimum of the local foreground
 emission where the DIRBE experiment aboard the Cosmic Background
 Explorer (COBE) will study the infrared background searching
 for the extragalactic background due to the redshifted emission
 of faint galaxies and protogalaxies.
 The proposed observations will be used in connection with
 the deep surveys done at longer wavelengths in the ISO
 central program and with the ground based surveys at
 shorter wavelengths (K band) for the interpretation
 of the DIRBE data.
 In the 3 to 6 microns wavelength range, the relative
 contributions of stars and the interstellar emission
 of very small particles for different types of galaxies
 is still an open question. The proposed survey will be
 necessary to remove (from the distant galaxies background)
 faint spiral and starburst galaxies at moderate redshifts
 for which the interstellar component could be dominant
 preventing a simple extrapolation from the K band surveys.