The objective of this study was to compare the effect of 2 levels of rumen-protected Met (RPM) from a prototype (72% D,L Met; Balchem Corp.) with a commercial product on plasma concentration and mammary clearance of Met and milk production in dairy cows. Twenty multiparous, lactating cows were used in a 5 × 20 Latin rectangle (period as rows and cow as columns) with 14 d periods. Cows were randomly assigned to the following treatments: 1) control (CTL; 0 g of RPM), 2) 6.0 g of MP-Met from Smartamine M (SM6), 3) 12.0 g of MP-Met from Smartamine M (SM12), 4) 6.0 g of MP-Met from RPM prototype (PT6), and 5) 12.0 g of MP-Met from RPM prototype (PT12). The basal diet was formulated for −9.5 g/d in MP-Met balance (2.02% of MP-Met supply), 16.0% CP, and 52% forage (corn and ryegrass silages) to 48% concentrate (corn grain and soybean meal). Treatments were top-dressed twice daily. Milk and blood samples were collected and analyzed. The RPM treatments increased (P = 0.01) milk protein percent compared with the CTL treatment. Relative to SM12, the SM6, PT6, and PT12 treatments did not differ in milk protein percent (3.19, 3.17, and 3.17 vs. 3.27%). Relative to CTL, the SM6, PT6, and PT12 treatments numerically improved milk protein percent. The SM6, SM12, and PT6 treatments showed no differences in milk fat percent. Relative to PT12, the SM12 treatment increased (P = 0.03) milk fat percent (4.70 vs. 4.42 ± 0.10%). Relative to SM12, the PT12 treatment tended to increase (P = 0.08) milk lactose yield (1.83 vs. 1.78 ± 0.03 kg/d). Relative to CTL, the RPM treatments increased (P = 0.01) Met concentration in coccygeal vessels and the abdominal mammary vein. Relative to RPM treatments, the CTL treatment increased (P < 0.01) mammary clearance rate of Met. In summary, the RPM treatments improved milk protein percent. The prototype at both levels sustained milk protein percent at the same level as SM6 and SM12. The RPM treatments increased plasma concentration and mammary capture of Met, which may have contributed to greater milk protein percent.