Litvak, N., Htoo, J. K. and de Lange, C. F. M. 2013. Restricting sulfur amino acid intake in growing pigs challenged with lipopolysaccharides decreases plasma protein and albumin synthesis. Can. J. Anim. Sci. 93: 505-515. Chronic subclinical levels of disease occur frequently in swine production and compromise nutrient utilization efficiency. Methionine and cysteine (M C) are involved in the pig's response to immune system stimulation (ISS), acting as substrates for the synthesis of compounds involved in the immune response, such as acute phase proteins (APP). This study was conducted to determine the impact of ISS with lipopolysaccharide (LPS) and M C intake (high vs. low; 4.0 vs. 2.2 g d^-1) on the fractional synthesis rate (FSR) of plasma albumin and fibrinogen, as well as protein in plasma and various tissues of starter pigs. Fifteen pigs (9.8±1.4 kg body weight) were allotted to one of three treatments: (1) Control (LPS- and high M C), (2) LPS and high M C, or (3) LPS and low M C. Pigs were given intramuscular injections of increasing doses of LPS 1 and 3 d before determining FSR using an intravenous infusion of a flooding dose of L-[ring-1H₅]-phenylalanine. Plasma levels of APP were not affected by the LPS challenge (P>0.10); only plasma albumin levels decreased with reduced M C intake (P=0.02). Total plasma protein FSR was increased during the LPS challenge (50.3 vs. 56.9% d^-1, SEM = 1.6; P=0.01). Albumin FSR was not affected by an LPS challenge (P>0.10), but decreased with reduced M C intake during the LPS challenge (49.5 vs. 41.9% d^-1, SEM = 1.6; P=0.005). Spleen protein FSR tended to increase during the LPS challenge (P=0.08). There were no treatment effects on protein FSR in liver, small intestine, loin or plasma fibrinogen (P>0.10). Restricting M C intake during a LPS challenge decreases albumin synthesis and tends to reduce plasma protein synthesis, implicating M C as important nutrients involved in the immune response.