Chronic subclinical levels of disease occur frequently in intensive swine production and compromise nutrient utilization efficiency. Sulfur amino acids (methionine plus cysteine; M+C) have been implicated in improving the animal’s response to immune system stimulation (ISS). Research objectives were to determine the effects of ISS on the optimal dietary methionine to methionine plus cysteine ratio (M:M+C) and on the fractional synthesis rate (FSR) of albumin, fibrinogen and total protein in plasma, liver, and small intestine (SI) of growing pigs. A nitrogen balance study showed that the optimal M:M+C was increased during ISS and greater than 0.62. In a flooding dose infusion study it was determined that total plasma protein FSR was increased during ISS and tended to decrease with reduced M+C intake. Plasma albumin FSR decreased with reduced M+C intake. The data implicates M+C as important nutrients involved in the immune response and careful dietary supplementation during ISS is necessary.