Recent research has demonstrated that supplementary sodium enhances the magnesium status of cattle fed fresh herbage by limiting the antagonism of potassium to magnesium absorption in the rumen (Chiy and Phillips, 1993). This may explain the observed reduction in the somatic cell concentration in the milk of grazing dairy cows when they are offered a sodium supplement (Phillips et al. 1996). Magnesium has been recognised as the main macroelement affecting immunocompetence in laboratory animals (Miller, 1985), and it is likely that it is important for grazing ruminants, which are prone to hypermagnesaemia. It is unclear whether sodium supplements would be beneficial to cattle fed conserved feeds, where hypomagnesaemia is less common.