Behavior of β-(1,3/1,6)-glucans (BG) of yeast cell-wall (Saccharomyces cerevisiae) in ruminal milieu was evaluated in vitro. The solubility of BG, pH, and production of VFA were measured after 4, 8, and 16 h of incubation in ruminal fluid. Two yeast cell-wall products were used: brewer’s yeast (BrYBG) and baker’s yeast (BaYBG), containing 15.1 and 26.3% DM of BG, respectively. Ruminal fluid was collected 3h post-feeding from a ruminally fistulated Holstein dairy cow fed a hay-based diet and strained through a metal sieve (2 mm). Within 45 min of sampling, 60 mL of ruminal fluid and 60 mL of degassed buffer solution (pH 7, 39°C) were added to 250-mL flasks, containing 5 g of yeast cell-wall product, flushed with O₂-free CO_2, and capped. For each incubation time, 5 flasks were prepared: 2 replicates each for BrYBG, and BaYBG, and one without product, used as control. Flasks were kept from light and air at 39°C in a waterbath rotary shaker. The pH was recorded at the start of incubation (0 h), 4 h, 8 h, and 16 h. Once the pH was measured, batch culture was centrifuged (4,500 × g for 10 min). Supernatants were taken to determine VFA contents. Total VFA produced was calculated by subtracting the initial total VFA concentration from the final concentration. The sediment BG content was determined (enzymatic kit, Megazyme). BG solubility was calculated by subtracting residual BG in sediment at the end of each incubation time from the starting concentration at 0 h. This batch culture experiment was repeated on 3 different days. At 4 h, the percentage of soluble BG was 3 times higher (P < 0.0001) for BaYBG than for BrYBG. At 16 h, it reached 96.4% and 92.5 for BaYBG and BrYBG, respectively. Amount of soluble BG increased (P < 0.0001) linearly with time (r² = 0.916 for BrYBG and 0.993 for BaYBG). Total VFA contents at 16 h differ (P = 0.0007): 95.2 vs. 110.7 mM, for BaYBG and BrYBG, respectively. The pH fall was less important (P < 0.0001) for BaYBG than for BrYBG. The decrease in pH and increase in total VFA production suggested that ruminal microflora would be potentially capable of degrading some soluble components in the yeast cell-wall products. But at 16h, 70% of DM BaYBG and 56% of DM BrYBG escape the ruminal degradation suggesting bioavailability in post-ruminal digestive tract.