In this study, an experimental challenge was carried out by feeding Nero Siciliano pigs with acorn to evoke melanin-like pigmentation and support the hypothesis that it is caused by ingested material. Twelve pigs were studied, 6 fed with acorns and 6 fed commercial feed. At slaughter, all the animals fed on acorns showed black discolouration of almost all lymph nodes. The lymph nodes were normal in size and shape. Histochemical tests performed on tissues allowed us to identify and differentiate the pigment. Immunohistochemical staining for macrophage markers showed macrophages containing a variable amount of melanin-like granules. Our data, and the well-known steps of melanin formation, confirm that swine enzymes could act on phenolic substances found in acorns.