Methionine (Met) is essential to support milk and milk protein synthesis. A study was conducted to evaluate the effects of a rumen-protected Met (RP-Met) prototype on milk production and plasma metabolite concentrations in dairy cows. Ten multiparous Holstein cows (76 ± 17 d in milk) were used in a 5 × 5 duplicated Latin square with 18-d treatment periods. Cows were randomly assigned to either a Met-deficient control diet (−16.4 g/d of MP-Met; −CTL), −CTL supplemented with Smartamine-M (28.7 g/d of RP-Met; SA), or 1 of 3 levels of a RP-Met prototype (Balchem Corp., New Hampton, NY) to provide 1.0 (31.6 g/d of RP-Met; 1.0Met), 1.25 (38.9 g/d of RP-Met; 1.25Met), or 1.5 (47.3 g/d of RP-Met; 1.5Met) times the amount of dl-Met supplied by Smartamine-M. The prototype contained 69% Met. Feed ingredients and milk and blood samples were collected and analyzed. Data were analyzed using the Mixed procedure of SAS 9.4 and reported as least squares means ± SEM. Compared with −CTL, SA, 1.25Met, and 1.5Met decreased (P = 0.02) lactose percent by 1.3%. Relative to −CTL, SA tended to increase milk protein content (P = 0.07). Relative to −CTL and SA, 1.0Met, 1.25Met, and 1.5Met treatments tended to maintain milk protein content. Relative to −CTL, SA and 1.5Met increased plasma Met concentrations (27.9 vs. 38.1 and 32.6 µM, respectively; P < 0.01). Compared with SA, 1Met, 1.25Met, and 1.5Met treatments support similar plasma methionine concentrations. In summary, SA and 1.5Met treatments increased plasma methionine concentrations. Smartamine-M tended to promote milk protein content; however, protein yield was not affected.