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Context: The counts of galaxies and AGN in the mid infra-red (MIR) bands areimportant instruments for studying their cosmological evolution. However, theclassic spectral line ratios techniques can become misleading when trying toproperly separate AGN from starbursts or even from apparently normal galaxies.  Aims: We use X-ray band observations to discriminate AGN activity inpreviously classified MIR-selected starburst galaxies and to derive updatedAGN1 and (Compton thin) AGN2 counts at 15 um.  Methods: XMM observations of the ELAIS-S1 15um sample down to flux limits~2x10^-15 erg cm^-2 s^-1 (2-10 keV band) were used. We classified as AGN allthose MIR sources with a unabsorbed 2-10 keV X-ray luminosity higher that~10^42 erg/s.  Results: We find that at least about 13(+/-6) per cent of the previouslyclassified starburst galaxies harbor an AGN. According to these figures, weprovide an updated estimate of the counts of AGN1 and (Compton thin) AGN2 at 15um. It turns out that at least 24% of the extragalactic sources brighter than0.6 my at 15 um are AGN (~13% contribution to the extragalactic backgroundproduced at fluxes brighter than 0.6 mJy).

AGN counts at 15um. XMM observations of the ELAIS-S1-5 sample