In order to observe the influence of low level exposure to lead on cellular immune functions in rats, sixty rats were randomly divided into six groups and were received diets with different levels of lead for eight weeks. Blood lead, serum tumor necrosis factor-alpha(TNF-alpha) level and the distribution of T lymphocyte subsets were measured. The pathological and ultrastructural changes of spleen were investigated. It was shown that lead exposure resulted in a significant increase of blood lead and decrease of the number of CD4 T-cells. But no significant difference was found in CD8 T-cell among all groups. Serum TNF-alpha at the lowest exposure group was significantly increased but that in other groups was lower than the control group. Electron microscopy for splenic immune cells indicated no significant difference between the lowest lead exposure group and the control group. The mitochondria of other groups were swollen, vaculated and pyknotic, endoplasmic reticulum were swollen and degranulated, nuclear chromatin were loosen. It was suggested that sub-acute exposure to lead, even at relative low exposure, would impair cell-mediated immunity and be harmful to spleen and thymus.