margins of safety must provide for those factors (Fig. 2) that may increase certain dietary vitamin requirements and for variability in inactive vitamin potencies and availability within individual feed ingredients. The NRC and ARC requirements often do not take into account that certain vitamins have special functions in relation to disease conditions and that higher than recommended levels are needed to obtain a response (Cunha 1985). In pigs artificially infected with Treponema hyodysenteriae, a causal agent of diarrhea, high supplementation with vitamin E (200 mg day-1) in combination with selenium (0.2 mg day-1) markedly reduced the number of pigs that became clinically ill (Tiege et al. 1978). In this study, clinical signs and pathological changes were less severe than in vitamin Edeficient pigs. Thus, high doses of vitamin E increase resistance against disease. For poultry, vitamin E feeding above NRC requirements has been shown to reduce the incidence of whole bird condemnation,