AbstractThe composition of mineral supplements provided for cattle by eleven well‐known manufacturers are presented. The principal products of each were divided into nine general‐purpose supplements, fifteen high‐phosphorus mixtures and eleven high‐magnesium compounds. The adequacy of the supplements has been considered on a basis of the amounts of each element provided when the supplements were fed according to the instructions of the manufacturer. Ten of the supplements (six manufacturers) were considered unsatisfactory in that they provided under 50% of the total phosphorus present in a gallon of milk when fed in a production ration as directed. On the same basis, nine of the supplements (the same six manufacturers) provided inadequate amounts of sodium chloride, but only two contributed insufficient calcium. Only one of the eleven high‐magnesium supplements would provide less than the equivalent of 2 oz. of MgO per day when fed as directed. Nine of the mineral supplements when included in 16 lb. of a production ration contributed under 50% of the total daily copper requirement of the animal and a further nine would have been inadequate if only 8 lb. of concentrates were fed. On the same basis, all the supplements provided adequate amounts of cobalt but only seven would substantially increase the total manganese content of the ration. In most of the cases where individual supplements were considered inadequate, this was due to low recommended rates of use, rather than to inherent defects in the chemical composition of the products.