Background: Endotoxin, widely present in the living environment of humans and animals, leads to endotoxemia during a short period. However, the long-term effects of endotoxin on immune function are unclear. Objective: To determine the importance of long-term endotoxin treatment on function of immune system. Methods: The mice were treated with different doses of lipopolysaccharide (LPS) for a month; the collected samples were then analyzed in terms of value changes in hematological parameters, lymphocyte subtypes, and immunoglobulins level. Results: The number of monocytes (MONO) and neutrophils (NEU) in the three treatment groups was significantly lower than the control after 30 days. However, the proportion of CD8+ T lymphocytes showed a rising trend in the mesenteric lymph nodes (MLNs) and Peyer's patches (PPs) while the CD4+ T cell was reduced. At the same time, a decrease was observed in the percentage of CD19+CD38+ B lymphocytes. Interestingly, the change of lymphocytes in PPs was more significant than that in MLNs, suggesting that immune response in the PPs occurred before the MLNs. Consistent with the changes in B cells, the content of IgA and IgG showed a downward trend. Conclusion: Long-term exposure to low-dose endotoxin had little or no effect on the immune function of the body, suggesting that the endotoxin can be rapidly eliminated by the immune system. Nonetheless, the number of immune cells was reduced in the high-dose group. T- and B-lymphocytes were significantly reduced, resulting in a decrease in immunoglobulin level, and showing a significant immune suppression state.