We aimed to determine the effect of consuming pure isolated micellar casein or pure whey protein isolate on rates of myofibrillar protein synthesis (MPS) at rest and after resistance exercise in elderly men. Healthy elderly men (72 (
sem1) years; BMI 26·4 ( sem0·7) kg/m2) were divided into two groups ( n7 each) who received a primed, constant infusion of l-[ ring-13C6]phenylalanine to measure MPS at rest and during 4 h of exercise recovery. Participants performed unilateral leg resistance exercise followed by the consumption of isonitrogenous quantities (20 g) of casein or whey. Blood essential amino acids and leucine concentration peaked 60 min post-drink and were greater in amplitude after whey protein ingestion (both, P< 0·05). MPS in the rested leg was 65 % higher ( P= 0·002) after ingestion of whey (0·040 ( sem0·003) %/h) when compared with micellar casein (0·024 ( sem0·002) %/h). Similarly, resistance exercise-stimulated rates of MPS were greater ( P< 0·001) after whey ingestion (0·059 ( sem0·005) %/h) v.micellar casein (0·035 ( sem0·002) %/h). We conclude that ingestion of isolated whey protein supports greater rates of MPS than micellar casein both at rest and after resistance exercise in healthy elderly men. This result is probably related to a greater hyperaminoacidaemia or leucinaemia with whey ingestion.