Abstract This study was conducted to examine the effects of synthetic lysine and methionine in pig diets on protein synthetic activity of liver tissue. Weaning pigs were fed 16% crude protein diets supplemented with 0, 0.1, 0.2 and 0.4 % solid or liquid lysine and 18 % crude protein diets as control in experiment 1, and fed 15 % crude protein diets supplemented with three levels of lysine-methionine (2:1 ratio) and 18 % crude protein diets in experiment 2. Protein synthesis was estimated by measuring the amount of incorporated [ 3H]-lysine into protein in liver acinar cell culture. Retained protein from liver tissue of pigs was significantly increased by supplementation of lysine, and solid form of lysine was more effective than liquid form. Also, the retained protein was significantly increased by supplementation of lysine together with methionine. In conclusion, the 2 % protein sparing effect in acinar cell culture of liver tissue was obtained by supplementation of the 0.2 % lysine and supplementation of lysine together with methionine was more effective to compensate the decreased protein synthetic activity by low protein level.