To evaluate the developmental immunotoxicity of brominated flame retardant, hexabromocyclododecane (HBCD) , maternal Sprague-Dawley rats were given HBCD at dietary concentrations of 0, 100, 1000, 10000 ppm from gestational day 10 to postnatal day 21 (postnatal week 3, PNW3). At PNW3 and PNW11, lymphocytes in the spleen, thymus, and peripheral blood of male pups were subjected to flow cytometric analyses for expression of surface markers (CD3, CD4, CD8a, CD25, CD45RA, CD71, and CD161 (NKRP1A)). The spleen and thymus weights, and number of white blood cells of two organs did not change between HBCD-exposed and control groups at PNW3 and PNW11. A significant decrease in thyroid hormone T3 and increase in serum albumin concentration were observed at PNW3 and lasted until PNW11. By flow cytometric analysis, the dramatic change was not observed in the population of the splenic and thymic T/B lymphocyte between the HBCD treated groups and control group. In the peripheral blood of BNW3 rats, the population of activated T cells was decreased and that of inactivated B cells was increased. And the population of NK cells in the spleen was decreased. All of these changes were mild in degree, and returned to the normal levels by PNW11. Production of anti-KLH IgG antibody after KLH immunization was reduced by the 10000 ppm HBCD treatment. These results suggest that developmental exposure to the highest dose of HBCD had a weak immunomodulatory effect at PNW3, and most of the immunomodulatory effect had recovered to normal levels by PNW11.