The cells in peritoneal cavities of mice were taken out and cultured in vitro. The amount to release H2O2 of the macrophages was assayed by fluorimetry. Polyporus umbellatus polysaccharide (PUP) could not only increase the number of macrophages and the amount of H2O2 release in the peritoneal cavities of normal mice, but also raise the lowered number of macrophages and the ability to release H2O2 in the peritoneal cavities of the mice with liver lesions caused by CCl4 significantly. So PUP could improve the cellular immunity of normal mice and the mice with liver lesions.