Abstract We studied the effects of maternal social stress induced by high density on offspring immunocompetence by experimentally altering population densities in field enclosures. We established high-density and low-density populations,with two replicates per density,in 4 enclosures (30 m ×50 m). F1 generation individuals born into the enclosures were taken to laboratory and housed in standardized conditions with ambient light and temperature until sexual maturity. We tested serum corticosterone concentrations of founders,and tested the serum antibody contents of anti-keyhole limpet haemocyanin (KLH)after injections with KLH antigen,spleen index and numbers of blood cells of mature offspring. We found that the founders of populations with high density had higher plasma corticosterone levels than those with low density. Offspring from high density populations had lower antibody contents of anti-KLH and greater spleen weight compared to the those from low densities. However,a significant difference in number of blood cells between offspring from different densities was not found. The results suggest that maternal social stress induced by high density can affect offspring’s immunocompetence in root voles,which may be play an important role in population regulation. In addition,based on ecological view,a concept of maternal stress effect is exploited.