Increased dietary zinc has been shown to reduce plasma total cholesterol in rabbits fed casein. However, the mechanism is not clear. The minerals associated with casein and soya protein are different and present in amounts which can alter the overall mineral composition of the diet. In particular, casein has a much higher ratio of zinc/copper than soy protein. Utilising a range of copper concentrations (2-80 micrograms/g diet) in 14 experiments with casein diets showed a linear relationship between the cholesterolaemic response and copper concentration (when both log transformed) in groups (n = 6) of rabbits fed for 12 weeks (r = -0.70, p less than 0.05). The lower the copper, the greater the response. In no case was the copper content deficient, indicating some interaction must be reducing its availability. Similar analysis utilising the range of zinc (10-125 micrograms/g) in the diets also showed an enhanced response at low intakes (r = -0.85, p less than 0.05). To elucidate the mechanism, 2 groups of rabbits were fed casein diets containing 21 and 68 mg zinc/kg diet for 12 weeks. Low density apolipoprotein B (LDL-apoB) turnover was carried out using 125I labelled LDL-apoB and data was fitted to the 2 pool model. The production of LDL-apoB was reduced in animals fed the higher dietary zinc. Zinc appears to act by enhancing the production of LDL-apoB in casein fed animals, perhaps by reducing the availability of copper.