Weanling rats were fed casein-based diets containing purified and mixed triglycerides to evaluate the effect of these lipids on mitogen-induced lymphocyte transformation, lymphoid organ weights, and fatty acid profiles of the total lipid in plasma, spleen, and thymus. Test lipids were added at a level of 8 g per 100 g of diet. All diets contained 0.82 g of safflower oil per 100 g. The digestibility coefficients for tristearin, tripalmitin, and trimyristin were 20, 37, and 85%, respectively. Digestibility coefficients for all remaining triglycerides were 90% or greater. The differences in mitogen-induced lymphocyte transformation among rats fed the various dietary lipids were unrelated to saturation of the lipid and correlated negatively with total lipid absorbed. Except for tripalmitin and tristearin, dietary lipids significantly altered the fatty acid profiles of the total lipids in plasma, spleen and thymus. It was concluded that the fatty acid profiles of the total lipid in plasma, spleen and thymus can be altered without accompanying major changes in mitogen-induced blood lymphocyte transformation. It was further concluded that mitogen-induced lymphocyte transformation was unrelated to saturation of dietary lipid and appeared to be associated negatively and weakly with the quantity of dietary lipid absorbed.