We evaluated the impact of dietary selenium intake on neonatal immune cell differentiation and function. A low selenium intake during pregnancy and lactation produced reductions in maternal plasma selenium (33%, p = 0.0001), milk selenium (36%, p = 0.001), and corresponding neonatal plasma selenium (47%, p = 0.008). Thymocytes from neonates receiving low-selenium milk showed an impaired activation in vitro (p = 0.001). The percentages of CD8 cytotoxic T cells (p = 0.03), CD2 T cells (p = 0.09), panB cells ( = 0.02), and natural killer cells (p = 0.07) were all decreased in neonates nursed by mothers fed a low-selenium diet. The results indicate that maternal selenium intake impacts neonatal selenium status which in turn influences the neonatal immune system development.