The athymic nu/nu mice represent a natural model for studying defense mechanisms. This study had the object of evaluating the phagocytic activity of polymorphonuclear leucocytes (PMN) and peritoneal macrophages (PM) in congenitally athymic homozygotic nu/nu mice. The results showed that the chemotaxis, random mobility, ingestion of latex particles and reduction of nitroblue tetrazolium were all normal. Nevertheless, the enzyme myeloperoxidase in the PMN was significantly diminished when compared with Balb/c mice. These results suggest a partial decrease in the oxidative metabolism of the PMN in nu/nu mice.