Sixteen diets based on straw, soya bean meal, maize and barley, were prepared with a commercial protected protein product, claimed to be of low degradability at rumen level and providing four levels of protein concentration in the diet (13.0, 15.5, 18.0 and 20.5 per cent CP) without or with three levels of protein protection (15, 30 and 45 per cent). A semi-continuous culture system adapted from the rumen simulation technique (RUSITEC) was used to compare the effects of the different levels of protein protection and inclusion on ruminal degradative and fermentative parameters. When looking into the protection of protein it seems from this experiment that for the lowest protection level (15 per cent) the diet must have at least 18 per cent of Crude Protein. If a higher protection level is pretended (30 or 45 per cent) and taking into consideration the degradative parameters, then it is better to use a 20.5 per cent level of protein inclusion. But if one wants also to optimise the microbial protein production, which is also available for intestinal absorption, then, and for the 30 per cent level of protection, the inclusion rate must be at least 15.5 per cent of protein when the degradation parameters are less affected. For the 45 per cent protection level the inclusion of protein must be 20.5 per cent when there is high microbial protein production and high rates of degradation but affecting negatively the fibre degradation rate.