The presence of these antibiotics as additives in the diet of ruminants has been prohibited in the European Union since 2006, given that their use is linked to developing microbial resistance to antibiotics. The main problem arising from the prohibition lies in the fact that the costs of production on farms have increased by an estimated 3.5% to 5%, besides increasing the environmental problems on the farms caused by higher emissions of methane gas. Thus, the research by Neiker-Tecnalia has aimed to find antimicrobial alternatives to growth-promoting antibiotics. Chitosan has turned out to be a viable alternative given that, apart from its proven antimicrobial activity, it has certain advantages being, as it is, a biodegradable, renewable and non-toxic compound. This antimicrobial activity enables putting forward the hypothesis that chitosan specifically affects some of the microorganisms of the rumen -the largest pre-stomach of the ruminants-, and in a beneficial way for rumen digestion. This effect is similar to that produced with monensin, the most widespread growth-promoting antibiotic employed in ruminant diet before its prohibition. The results obtained from the research were very promising as regards the potential of chitosan for handling ruminant fermentation – the goal being to improve nutritional efficiency and boost the use of nutrients for ruminants. If these improvements turn out to be commercially productive and competitive, chitosan could well represent an alternative effective growth-promoting antibiotic for use in the diet of ruminants. For the research a series of experiments were put forward to determine the effect of chitosan on ruminant fermentation. Initially, simulation trials were undertaken to determine the effect of different types of chitosan on ruminant fermentation, as well as the influence of the diet, the dosage and the incubation time on these effects. For the first trials, a technique of discontinuous cultures was employed. In this technique a specific feed item is incubated with an inoculum from the rumen of sheep, under conditions of pH and temperature that simulate those of the rumen itself. The trials showed that the chitosan modified the ruminant fermentation in a manner that could turn out to be beneficial for the animal; for example, with increments in the production of propionic acid during the fermentation of up to 95%. Moreover, it was observed that these effects or modifications were more important as the degree of deacetylation of the chitosan molecule increased and as the ration or diet contained a greater proportion of concentrate. In the second round of trials the Rumen Simulation Technique or RUSITEC was used. This is different from the previous technique in that in this case semicontinuous fermenters were used in which there is a recycling of the culture medium. It thus enables maintaining a culture of stable ruminant microorganisms for longer periods in order to undertake studies for a longer time and thereby observe the persistance of the effects observed with the previous technique, and enable discarding possible adaptive effects of the ruminant microorganisms to chitosan. In these trials the effect of chitosan on rumen fermentation was observed to persist during the incubation time, thus enabling them to discard adaptive effects of the ruminant microorganisms to this compound. In this way, trials obtained highly promising results, based on an improvement on the energy efficiency of the rumen fermentation process with fermentations aimed at the production of propionic acid and a reduction in the production of methane during this process of up to 50%. Experiments with animals were carried out a posteriori. In these trials it was shown that the modulator effect of rumen fermentation that had been observed in vitro was also observed when the chitosan was administered directly to the animals, obtaining improvements in energy efficiency of the rumen fermentation process without it affecting the digestibility of the ration.