In order to study the protein sparing and growth promoting effect of mecadox for growing pigs, a total of 160 three way crossbred pigs (Landrace x Duroc x Large White) weighing approximately 11 ㎏ initially were used for a period of 12 weeks of feeding trial and metabolism trial. Experimental diets of two different level of dietary protein (high-19% or low-17% for weaned pig and high -17% or low-15% for growers) were fed with (50 ppm) or without (0 ppm) mecadox (carbodox : methyl- 3 - (2 -quinoxalinylmethylene carbazate-N¹, N⁴-dioxide)). (1) Although there was no statistically difference, pigs fed diets with 50 ppm of mecadox gained faster and more efficient than pigs fed unsupplemented diets regardless of protein level in the diet. Pigs fed the high protein level supplemented with mecadox resulted in best effects of gain and feed efficiency. (2) Rate of gain and feed efficiency of pigs fed the high dietary protein level of 19 % in weaning diet. and 17% in grower ration unsupplemented with mecadox were similar to pigs fed the low dietary protein level of 17% in weaning ration and 15% in grower ration supplemented with mecadox. The growth promoting effect of mecadox was remarkable for younger pigs than growers. (3) Digestibility of crude protein and crude fat was slightly improved and that of crude fiber was decreased when mecadox was added to growing diets. It was apparent that nitrogen retention rate of pigs receiving mecadox was slightly higher than unsupplemenred pigs. Based on the above results, it may be concluded that the addition of mecadox at 50 ppm level could improve the growth rate and feed efficiency from weanling pig to growing pig and that mecadox might have protein sparing effect up to 2% of total dietary protein for growing pigs.