It is commonly assumed that the weight of the amino acids in a parenteral amino acid mixture equals the amount of protein they provide. This assumption ignores the fact that the molecular weight of free amino acids is 18 mass units greater than when they are protein bound. The actual amount of protein substrate provided by commonly used free amino acid mixtures was determined by analyzing the amino acid composition of 3 commonly used parenteral amino acid solutions and the proteins that would be formed from them, and comparing the results with similar data from 3 nutritionally important proteins. After correction for hydration status, the ratio of essential amino acid mass to total mass of the amino acid mixtures was similar to albumin, myosin, and actin. However, all of the amino acid mixtures provided 17% less protein and energy than is now widely assumed. Current parenteral nutrition guidelines recommend 0.8-1.5 g mixed amino acids/kg normal weight per day, on the assumption that they are equivalent to formed proteins, but they are not equivalent. Clinicians who aim to provide 0.8-1.5 g protein/kg must administer 1.0-1.8 g mixed amino acids/kg.