Protein concentration in human from 39 well-norished American women and its adequacy for growth of exclusively breastfed infants(BF) and breastfed infants fed supplementary foods(BFS)from 1-6 months postpartum were studied. Mean protein concentration of breast milk measured by Lowry et al., using human serum albumin as a standard, over the first 6 months lactation was 1.31$pm$0.13g/dl. Concentration of protein was singnificantly higher at the first month of lactation (1.55$pm$0.23g/dl)(P<0.05) than any other month studied. Mean volume of breast milk ranged from 662-848ml/day in the BE group and from 415-661ml/day in the BFS group during the first 6 months of lactation. Mean protein intake of infants ranged from 1.3-2.2g/kg in the BF group and from 1.4-2.1g/kg in the BFS group. Mean protein intake (g/kg body weight) of both BF and BFS groups was less than Recmmended Dietary Allowance(1989, USA) of 2.2g/kg except at 1 month of age. However, mean growth of the infants was normal according to NCHS reference, suggesting that the RDA for protein was unrealistically high for infants during 2-6 months of age. Protein provided by breast milk alone appeared adequate for normal growth during this time.