The protein quality of the liquid diet Two Cal HN was evaluated by plasma amino acid (PAA) ratio and by chemical score. The PAA test was performed in young healthy men, elderly healthy men, and elderly institutionalized, tube-fed men. In all three groups the PAA ratio for tryptophan was negative, indicating a deficiency of this amino acid in the formula's protein. But the amino acid score for tryptophan based on the manufacturer's stated amino acid composition was adequate (0.90). To resolve the discrepancy between PAA test and amino acid score, the formula's protein was isolated and its amino acid composition analyzed. In three samples of Two Cal HN, the tryptophan content of the isolated protein averaged only 45% of the content stated in the manufacturer's product description. A similar discrepancy was then found for Isocal, another liquid diet used in nursing homes.