To investigate the effect of lanthanum nitrate on immune development of offspring rats, which exposed to pregnant rats, by measuring the level of interferon, interleukin, and the proliferation of spleen T lymphocyte, spleen B lymphocyte and natural killer (NK) cells. Methods Pregnant SD rats were randomly divided into control group, low dose group, middle dose group and high dose group,with 10 rats in each group. The rats were treated with lanthanum nitrate at of 0,2, 20, and 60 mg/kg BW respectively, from gestation day 7 to gestation day 16. On postnatal day 4 (PND4), the number of rats was reduced to 4(2 male and 2 female per litter), about 160 rats in the four groups. One male and one female from each litter were chosen as A queue, and the other one male and one female were B queue. On postnatal day 52 (PND52), rats of the A queue were observed including body weight, peripheral blood lymphocyte typing and serum γ-interferon (γ-IFN), interleukin-2 (IL-2), IL-4, IL-1α, IL-10, and tumor necrosis factor (TNF) content. The rats of B queue were detected T-dependent antigen response and lymphocytes proliferation on postnatal day 55 (PND55). Offspring rats were weighed every week during the experiment. Results There was no significant difference between the experimental groups and the control group (P>0.05), including the relative weight of liver, kidney, lung, brain, spleen and thymus. Spleen T lymphocyte proliferation in male rats of the high dose group was higher than the control group (P<0.05), with a dose-response relationship. The number of blood NK cells in female rats of the experimental groups were more than the control group (P<0.05) with, a dose-response relationship. The results show that benchmark dose lower bound was 0.21 mg/kg BW. Conclusion Exposure to lanthanum nitrate during pregnant period had a slight stimulation on NK cell and spleen T lymphocytes, however, there was no significant toxicity on the immune system development of rats offspring.