Abstract: Two trials were carried out to study how a compound formed by natural extracts from Rutaceae plants, cinnamon oil and organic acids (RUTEX® PG) could have an effect on intestinal microorganisms on piglets. To obtain this synergistically combination first of all natural extracts and organic acids were separated individually to see its inhibitory action on bacterial growth in vitro. Secondly, the aim was to search the best natural extract and organic acid combination between all the possible ones. RUTEX® PG was the combination that was used in vivo trials with piglets. To study RUTEX® PG effects on piglets two trials were designed. The trials consisted in a comparison between the effect of adding RUTEX® PG to the feed on the intestinal microorganisms of piglets and the effect of antibiotics in feed. The in vitro experiments results showed a synergistic effect of the Rutaceae plant extract and the organic acids tested (citric, formic, lactic and propionic) against different microorganisms: growth inhibition was observed for the organic acid-natural extract combination, while the same concentrations did not show any inhibition when the products were tested separately. Inhibitory activity for cinnamon oil was not tested, because it was added in a very low concentration just to confer an aromatic effect. It was observed, as well, lower bacterial counts at the intestinal segments (duodenum, jejunum, ileum and colon) in comparison with the negative control group. The conclusion is that we can say RUTEX® PG can be active all along the gastro-intestinal segments.