The sulfur AA (SAA; methionine and cysteine):Lys ratio for protein accretion is 55% for 7 to 11 kg pigs (NRC, 2012). The use of SAA to support immunity in addition to accretion of lean tissue may exert additional requirements in immune stimulated pigs. Moreover, due to negative whole body protein balance resulting from muscle wasting during an immune challenge, performance responses may not be suitable response criteria for determining SAA requirements. A study was conducted to determine the standardized ileal digestible (SID) SAA:Lys ratio of weaned pigs under an enterotoxigenic Escherichia coli (ETEC) challenge using gene expressions of methionine adenosyltransferase 1 and 2-α (MAT1A and 2A), 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), and cystathionine γ-lyase (CTH) as response criteria. Thirty-five [Duroc × (Yorkshire × Landrace)], 7-kg piglets were assigned to 5 dietary treatments in a completely randomized design. The corn-wheat-soybean-meal based antibiotic-free diets had graded SID SAA:Lys ratios (48, 54, 60, 66, and 72%) and 1.18% SID Lys. Feed was offered at 4% BW and fed twice a day at 08.00 and 16.00 h. Piglets had a 6-d diet adaptation period and were all orally challenged with 6 and 15 mL of ciprofloxacin-resistant ETEC K88+ (5 × 10⁹cfu/mL) on d 7 and 10, respectively. Blood samples were collected before and after challenge. On d 13 all pigs were killed to collect liver and ileal tissues for gene expression of MAT1A and 2A, MTR, and CTH using quantitative real-time PCR. Serum TNF-α concentration 6 h after inoculation was higher (P < 0.05) than pre-challenge and was improved with increasing SAA:Lys ratio. Data were subjected to ANOVA using Proc Mixed of SAS 9.2. Increasing SAA:Lys ratio linearly decreased (P < 0.01) liver MTR, while quadratically increasing (P < 0.01) CTH and MAT1A gene expression. Ileal expression of MTR and MAT2A linearly increased, whereas, CTH and MAT2A were quadratically increased (P < 0.05) with increasing SAA:Lys ratio. Maximum transmethylation, transulfuration, and remethylation was at SAA:Lys ratio of 60% indicating that this ratio was enough to support piglets during an immune challenge.