The study evaluated dietary supplementation with live yeast (LY) Saccharomyces cerevisiae (CNCM I-4407, 10(10)CFU/g, Actisaf; Phileo Lesaffre Animal Care, France) on rumen fermentation and serum metabolic profile in lactating dairy cows. Fifty Holstein cows received a total mixed ration with (Live Yeast Diet, LYD, n=25) or without (Control Diet, CD, n=25) 5×10(10)CFU/cow/day of LY from 3 to 19weeks of lactation. Rumen fermentation and serum metabolic profile were measured in eight cows per treatment at 3, 7, 11, 15, 19weeks post-partum. LYD showed an increased daily milk yield (+4%) over CD (p