We have observed and studied the immune response, ultrastructure and phagocytosis of peritoneal macrophages (M?) of mice in protein deficiency by means of indirect fluorescent antibody test (IFAT), immunoenzymatic staining technique (IEST),fluorescence isothiocyanate antibody (FITC-Ab) quantitative assay, M? phagocytosis test, scanning electron microscopy (SEM) and im-munoelectron microscopy (IEM).The results showed that the body weight of mice was continuously declined after fed protein deficient diet. In the same time fluorescence reaction and enzyme stain on the M? surface was retarded. The amount of FITC-Ab on the M? membrane was decreased. The villi on the M? surface were shortened, the positive rates and positive degree of cells were lowered,the reaction of cell membrane and nuclear membrane was retarded in SEM and IEM.The phagocytic function of M? was inhibited.The results showed that in protein deficiency, the immune reaction, structure and function of peritoneal M? of mice were markedly affected.