Abstract: The invention relates to a method of inhibiting the biofilm formation by thermophilic adhering microbes of paper and board machines on the surfaces of paper or board machines, and/or removing such biofilms from the said surfaces by adding to the circulation waters of the paper or board machines at least one pure substance isolated from a plant or at least one plant extract or a mixture thereof in such a concentration which is effective against thermophilic adhering microbes. The invention further relates to a method of determining the need of the addition of an anti-biofilm agent in paper and board manufacturing processes, and an assembly kit suitable for the same.