Experiments were conducted to evaluate the availability to ruminants of lysine from hydroxymethyl lysine, a product potentially resistant to ruminal degradation yet able to release free lysine when subjected to the acidic environment of the abomasum. An in vitro ruminal fermentation assay that led to ammonia production from free lysine was used for initial assessments, but the hydroxymethyl lysine was inhibitory to lysine degradation at the concentrations tested in vitro; therefore, an in vivo assay with sheep, using plasma lysine concentrations as the response criterion, was used for assessment. twelve mature sheep were fed graded amounts of lysine from either a commercially available ruminally protected lysine product with known availability or from hydroxymethyl lysine. the protected lysine product provided 3 or 6 g/d of metabolizable lysine, whereas the hydroxymethyl lysine provided 3 or 6 g/d of total lysine. Plasma lysine concentrations increased linearly in response to both the ruminally protected lysine product and hydroxymethyl lysine. by slope ratio analysis, the bioavailability of lysine in hydroxymethyl lysine was estimated to be 94% of that for the commercially available product. We concluded that hydroxymethyl lysine may be used as an effective means of supplementing lysine to ruminants.