The aim of this paper was to give a state of the art description of chitosan as a wood protecting agent. Chitosan is a metal free natural compound derived from crustacean shells and is under evaluation as an environmentally benign wood protecting agent. Information from journals states that chitosan may act both fungistatically and at higher concentrations, as fungitoxic, but the mode of action is not yet fully understood. The hypothesis with most support in the literature is that chitosan interacts with the cell membrane and causes alterations in permeability. It is not proven that chitosan is more effective against a particular class of fungi, but morphological changes and reduction in growth rate is reported from a range of fungi. Results from agar plate growth rate tests are not conclusive with respect to whether high or low molecular weights are most effective against micro-organisms. Other factors than the molecular weights may influence microbial activity of the species studied, i.e. FA, pH, and internal distribution of the monomers, concentration and additives. In results available in the literature it is obvious that there is a dose-response relationship between chitosan and antimicrobial activity. In agar plates a lethal/totally inhibiting concentration is usually between 0.1 and 1 %. Chitosans in solution are more effective against antimicrobial growth than chitosans in suspension (i.e. solid chitosan particles). This is further reflected in that higher concentrations of chitosan are needed in wood than in agar amended plates. If the treated wood is subjected to leaching, around 5 % (w/v) chitosan solutions seems to be needed for good efficacy against fungal decay. Some tests where chitosan fails in decay tests are probably because of the use of to low concentrations, or to low penetration of chitosan solution due to high molecular weight.

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