In tanneries, microorganisms are provided with environments suitable for growth. The proteins and fats in the hide are ideal nutrients for bacterial and fungal growth. In particular, pickled pelts, wetblues, and vegetable-tanned moist leathers are prone to mold growth when stored or shipped. In the past, chemical preservatives were considered primary protection, but recent years have brought concern over potential ecological hazards. These concerns have resulted in the replacement of pentachlorophenol preservatives in most tanneries in Europe with modern heterocyclic fungicides. The analysis of fungicides on wetblues shows the penetration behavior of the heterocyclics throughout the pelt. The advantages of the combination of penetrating and surface-active materials are stressed.