Microglia are monocytes (MC) that function as resident macrophages in the central nervous system (CNS). The MC are the most important immune defense in the CNS, responding rapidly to infection in part by releasing pro-inflammatory cytokines and reactive oxygen and nitrogen species (ROS, RNS). Methionine (Met) is the first-limiting amino acid for milk protein synthesis and also serves as substrate for the antioxidants glutathione and taurine through the 1-carbon metabolism and transsulfuration pathways. Previous research demonstrated beneficial effects of enhancing Met supply during the periparturient period on voluntary DMI, inflammatory and oxidative stress status, and innate immune function. Because of the linkage between inflammation, oxidative stress, and satiety centers in the brain, we sought to evaluate the in vitro response of MC to incremental doses of Met. Immortalized ovine MC cultures (n = 5/treatment) were pretreated with 0, 10, or 20 µM Met for 12 h followed by oxidative stress challenge with 150 µM H₂O₂ for 3 h. An ANOVA using the MIXED procedure of SAS was used in the statistical analysis. Concentration of ROS was greater overall (P < 0.05) with Met supplementation and 20 µM Met prevented an increase in ROS after challenge with H₂O₂. Although MC not pretreated with Met had greater (P < 0.05) IL-6 concentration in culture medium, Met supply prevented an increase in IL-6 even after H₂O₂ challenge. Similarly, compared with MC not pretreated with Met, cellular proliferation assayed with Resazurin was lower (P < 0.05) with pretreatment of 10 or 20 µM Met. Challenge with H₂O₂ markedly increased (P < 0.05) cellular proliferation to the levels detected in MC not pretreated with Met. Indirect measure of NADPH oxidase activity via Resazurin assay indicated that, compared with MC not pretreated with Met, 10 or 20 µM dampened (P < 0.05) metabolic activity of MC when challenged with H₂O₂. Overall, data indicate that enhanced Met supply to MC can help control ROS production, cellular proliferation, and metabolic activity even during oxidative stress. The functional outcomes of these responses in vivo remain to be determined.