Lysine and methionine have been identified as the two amino acids most limiting milk production in lactating dairy cows. Presently there are no rumen protected lysine (RPL) products commercially available that have reported in vivo lysine bioavailability. The objective of this study was to determine the plasma lysine response to two RPL products and compare with a plasma lysine response curve developed from known amounts of abomasally infused lysine. This approach was recently validated as a reliable approach for determining lysine bioavailability. The two RPL products (RPL-1 and RPL-2) were protected by proprietary lipid encapsulation (Balchem Corporation) and contained 47 and 46% lysine-HCl. In vitro rumen testing of both products indicated a rumen bypass of 75 and 87%, respectively (estimated passage rate of 11%/h). The study was designed as two consecutive 4 X 4 Latin square experiments and used 4 ruminally fistulated Holstein cows. In the first Latin Square, 0, 25, 50 and 75 g/d of raw lysine-HCl were abomasally infused for 3 d periods and plasma samples were used to generate a blood lysine response curve. In the second Latin Square, RPL-1 and RPL-2 were fed each at two doses (50 and 100 g lysine-HCl) for 7 d periods. Throughout the study cows were fed a TMR diet that contained 19% CP to ensure all amino acids, including lysine, were not limiting. Blood was sampled every 2 h over the last 24 h of each period and amino acids were determined by isotope dilution using a GC-MS. Abomasal infusion of graded levels of lysine resulted in a linear increase in plasma lysine concentration (P< 0.001; R2 = 0.72) with an intercept of 68 μM (SE = 4.2) and slope of 0.52 μM per gram of infused lysine-HCl (SE = 0.09). Based on this response curve and the plasma lysine concentration when feeding RPL the bioavailability of RPL-1 and RPL-2 (averaged across both RPL doses) was estimated to be 46 and 56%, respectively, but did not differ significantly from each other (P = 0.62). The results from this study indicate that this lipid encapsulation provides a means to effectively supply bioavailable lysine to the lactating dairy cow.