Mechanically induced polymorphic transformation of polymorph III of carbamazepine into form IV was accelerated and brought to completion by contaminating deliberately a milling jar and ball by touching them with fingers. In model experiments with additives of water and fatty acids (important components of fingerprints), carbamazepine dihydrate and carbamazepine form II, respectively, were obtained, without any evidence of form IV, as a result of mechanical treatment under otherwise the same milling conditions. A possible interpretation was suggested.