Serum transferrin is a sensitive indicator for the establishment of a subclinical protein deficit. 2. When a diet moderately poor in protein was given (0.5-0.6 g/kg body weight/daily) was given the transferrin values (243 +/- 61 mg/100 ml) were significantly lower than in the control group (284 +/- 61 mg/100 ml), but they were altogether still in the lower normal region.