The authors thank all the members of the Human Nutrition Laboratory, and are grateful to Françoise Morel-Laporte, Noëlle Mathieu, Marion Brandolini and Guy Manlhiot for their skilfultechnical assistance. The authors especially thank the volunteers who participated in this study ; Background/objective: Adequate protein intake is crucial to maintain muscle protein content in elderly subjects, but quality of dietary proteins should be considered. The aim was to determine whether soluble milk protein offers an original strategy to increase muscle anabolism in elderly subjects via a synergistic effect of fast-digesting proteins together with a unique essential AA content. Design: We investigated the effect of a 10-day adequate-protein (AP) or high-protein (HP) diet together with the protein source as caseins (CAS) or soluble milk proteins (PRO) on specific muscle protein fractional synthesis rates (FSRs) in healthy elderly men (71.8 +/- 2.4 yr, n = 31). The isotopic study consisted of two periods of 4 h each: a post-absorptive and a postprandial period. The fed state was defined by consumption of either 15 g or 30 g of PRO or CAS, given fractionally every 20 min for 4 h. Soluble milk proteins are produced using a membrane process directly from pasteurized milk. Measurements: Specific muscle protein FSRs were measured during both postabsorptive and postprandial period using a continuous infusion of L-[1-C-13]leucine. Results: FSR of sarcoplasmic muscle proteins and actin did not increase significantly in the postprandial state compared to postabsorptive state, whereas myosin FSR rate was increased by feeding whatever the protein source in HP groups (0.024 +/- 0.005 vs 0.053 +/- 0.011% h(-1), P