Taste detection thresholds for 19 L-amino acids and four monohydrochloride derivatives were determined for young and elderly subjects. Although the ranges of the thresholds were extensive, the elderly were found to have significantly diminished sensitivity. On the average, thresholds were 2 1/2 times higher in the elderly than in young subjects. Relationships between the taste detection thresholds of amino acids, their chemical structures, and suprathreshold taste qualities were found. Amino acids with aliphatic side chains and those containing hydroxyl groups and having sweet tastes at suprathreshold concentrations tended to have the highest thresholds. Amino acids containing sulfur, as well as those with acidic or basic groups, tended to have the lowest thresholds.