A total of 260 tons of a new mineral feed supplement [MKP-C(P)] was used on farms in the Písek district. In the period from January to April, 1988, the mortality of calves decreased by 2.9% in comparison with the same period of 1987 when the Polymin Z or MKP-C supplements had been used. The MKP-C(P) supplement, containing 8 to 10% calcium, 3 to 4% phosphorus, 6 to 7% magnesium and 8 to 10% sodium, was used in all first-calver barns in the 1987/88 winter feeding season. It was administered to about 7300 highly pregnant heifers and first-calvers at rates of 0.15 to 0.2 kg per head and day. It has been concluded from the results of the experimental and farm-scale testing of the MKP-C(P) supplement, containing much more magnesium (by 180%) and less phosphorus (by 46%) than the commercially produced supplements MKP-C and Polymin Z, that the new product is able to meet all the phosphorus requirement, together with a rational management of magnesium, in young breeding cattle. Apart from a higher effect, especially in the compensation of hypomagnesiaemia and metabolic acidosis, the use of the new supplement in first-calver stocks allows to reduce the total financial costs of mineral feed supplements by 20%. The basis for the reduction of costs is the saving of phosphates in the potato-growing areas; this is also associated with a reduction of the supply of cadmium to the cattle feed rations.