The biodeterioration or biological destruction of old calf and cow leather samples has been studied. Long term testing with different types of leather kept in small high RH chambers led regularly to the development of various kinds of fungi and actinomyeceten. The roll of these fungi and actinomyceten in the deterioration of the individual components of the leather samples was examined. It has been determined that the micro-organisms are in the position to destroy the tannins and water insoluble substances in the leather samples. Furthermore, there occurred an enzymatic disintegration of the collagen fibers, which led to an increase of the water soluble collagen derivative and to total destruction of the thinner types of leather. The more resistant thicker types of leather were not entirely destroyed.