Candidacidal and phagocytic tests were performed on 34 female sheep fed high sulphur diets containing varied concentrations of thiamine, copper, and molybdenum for 14 weeks. Tests were conducted at weekly intervals for five weeks during the latter half of the experiment. The ability of polymorphonuclear leucocytes from sheep on diets unsupplemented with thiamine to kill phagocytosed Candida albicans was lower than those on diets supplemented with thiamine. The ability of polymorphonuclear leucocytes to phagocytose C albicans was lower in animals on low copper diets than in animals on high copper diets. Factorial analysis showed that thiamine supplementation significantly increased (P less than 0.0001) candidacidal but not phagocytic activity. By contrast, copper supplementation significantly increased (P less than 0.0005) the phagocytic activity with no effect on candidacidal activity. The observed detrimental effect of high sulphur intake on in vitro polymorphonuclear leucocyte function can mean that ruminants in areas where large quantities of sulphur are taken in with water and feed have compromised immune function due to lower copper and thiamine status and hence are at risk of increased susceptibility to infections.