Materials from agroresources (granulates and fibres) are more and more developed in order to use renewable resources and to limit carbon footprint. These materials are used in many applications like building constructions. They are exposed to very high variations of temperature and humidity... In some cases, these conditions may be favorable for the development of microorganisms. Vegetal granulates or fibres are composed by a set of polysaccharides whose nature and function are different. Some microorganisms may use this sugar for growth. Microorganisms and more precisely mold growth may alter materials by consumption of constituent and induce health damage due to the liberation of spores in the atmosphere. In our study, we focus on the impact of microorganisms on agroresource materials. First, we only concentrate on mold growth on flax fibres. We have tested the evolution of chemical composition of lignocellulosic materials.