Weanling mice were fed ad libitum from age 23 to 37 days either an 18 or an 0.6% protein diet. Half the animals in each dietary group received supplemental triiodothyronine (T3, 0.2 mg/kg diet). T3increased the primary in vivo antibody response of protein‐deficient mice to sheep red blood cells, as measured by both splenic plaque‐forming cells (PFC) per 106nucleated spleen cells and serum hemagglutinin titers. T3also increased PFC/spleen in well‐nourished mice. The effect on protein‐deficient animals was achieved although nutritional status in these animals, as estimated by weight loss and carcass composition, was further impaired by T3supplementation. These results support the hypothesis that immune functions can be improved independendy of nutritional status in severe (wasting) malnutrition. Insofar as T3was effective in a model of malnutrition that does not reduce serum total or free T3levels, the phenomenon appears to represent a pharmacological action of the hormone.—Perry, K. J.; Filteau, S. M.; Woodward, B. Dissociation of immune capacity from nutritional status by triiodothyronine supplements in severe protein deficiency.FASEB J.2: 2609‐2612; 1988.