A high phytic acid diet (barley, wheat, soya bean meal, 4 g P/kg, of that 2/3 phytate P) without added phytase, with phytase supplement (1000 U/kg diet) or with supplementary phosphate (2,2g P/kg diet) was examined with 3 × 12 weaned piglets. The high dietary P level due to phosphate addition, significantly improved body weight gain and feed:gain ratio. The phytase effect on these criteria was small. The phytase but more the supplementary phosphate increased P concentration of serum. In case of phytase addition pigs had 10% less ash P and Ca in the rib than animals of phosphate group. In the group without phytase or P addition the ash, P and Ca concentration of bone were decreased by 20%.