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It is widely accepted that purine bases (PB) could overestimate microbial production when used as microbial markers as result of rumen by-pass of dietary PB. Free nucleic acids are rapidly degraded when incubated in rumen content either in vitro or in vivo (McAllan and Smith, 1973) and nylon bag disappearance of food PB seems to confirm their low contribution to duodenal flow (Pérez et al. 1996). However isotopic labelling of microbial PB (Pérez et al. 1997) have resulted in much higher estimates of non microbial PB arriving to duodenum, which were attributed to differential uptake of dietary PB by bacterial and protozoal populations leading to lack of representativeness of the reference bacterial sample. The present experiment aimed to know if estimation of microbial contribution to duodenal PB could be biased by differences between rumen bacterial and protozoal populations in 15N enrichment of PB.