The effect of megadoses of vitamin E was studied in 13 adult males and five young boys. Three hundred milligrams of vitamin E as dl-alpha-tocopheryl acetate, given daily for a period of 3 weeks produced a significant depression in the bactericidal activity of the leukocyte and the mitogen induced lymphocyte transformation. The delayed hypersensitivity of the skin to phytohemagglutinin was, however, not affected by the supplementation. The implication of the study and the discrepancy between the in vitro and the in vivo results of the cell-mediated immunity are discussed.