Two experiments were conducted using the technique to determine the bioavailability of Lys in rumen-protected (RP)Lys supplements manufactured by QualiTech Inc. Bioavailability estimates are calculated by dividing the slope of the regression line relating changes in plasma Lys concentrations from feeding an RPLys supplement by the slope of the regression line obtained by continuous abomasal infusion of Lys. Experiment 1 examined the efficacy of the company’s first generation product. Experiment 2 was to determine the effect of changes in core/matrix composition and amount of encapsulation on Lys availability. Six multiparous ruminally-cannulated Holstein cows were used in Exp. 1 in a replicated 3 × 3 Latin square (193 DIM). Treatments were 1) 0 g/d Lys, 2) 60 g/d infused Lys from Lys-HCl (80% Lys), and 3) 60 g/d fed Lys from RPLys (56% Lys). Five multiparous ruminally-cannulated cows were used in Exp. 2 in a 5 × 5 Latin square (108 DIM). Treatments were 1) 0 g/d Lys, 2) 60 g/d infused Lys from Lys-HCl (80% Lys), 3) 60 g/d fed Lys from product A (30.4% Lys), 4) 60 g/d fed Lys from product B (47.4% Lys), and 5) 60 g/d fed Lys from product C (37.3% Lys). Before feeding, the products were mixed with 2 kg of the Lys-adequate basal TMR, placed in ziploc bags, and stored for 8 h at 4°C. Fed treatments were offered to cows 30 min before each of 3 daily feedings. Fed treatments not consumed after 15 min were placed in the rumen. Tail vein samples were collected 2, 4, 6 and 8 h after the morning feeding the last 3 d of each period. Deproteinized plasma samples were composited by cow/d and stored (−80°C) until AA analysis. Lysine concentrations (µM) were expressed as a proportion of total AA before regression analysis. Data were analyzed using the PROC MIXED and REG procedures of SAS. Lysine was the only plasma AA that increased linearly in response to treatments (P < 0.02). Calculated Lys bioavailability estimates were 27.1% in Exp. 1, and 17.0, 24.5, and 28.5%, respectively, for Products A, B, and C in Exp. 2. The technique was sensitive enough to confirm differences between Product A vs. products B and C in Exp. 2.