The international 1985 FAO/WHO/UNU upper dietary requirement for lysine of 12 mg.kg-1.d-1 may be inadequate for healthy Indian adults. To test this, we used a modified indicator amino acid oxidation technique to assess the adequacy of lysine intake of 12 and 28 mg.kg-1.d-1. Seven healthy, male, Indian subjects were studied during each of two randomly assigned 6-d periods while receiving an otherwise adequate diet based on an L-amino acid mixture. Beginning at 1800 on day 6 of the diet, a 24-h infusion protocol in which a [13C]leucine tracer was administered intravenously was used to assess leucine oxidation and daily leucine balance at each test lysine intake. Mean 24-h leucine oxidation was 54.7 compared with 46.9 mg.kg-1.d-1 (P < 0.05) and mean 24-h leucine balances were -4.1 and 3.5 mg.kg-1.d-1 (P < 0.05) for lysine intakes of 12 and 28 mg, respectively. Leucine balances were significantly negative (0.025 < P < 0.05) with the 12-mg lysine intake and not significantly different (P > 0.10) from zero or equilibrium with the 28-mg intake. These findings indicate that the international requirement for lysine appears to be inadequate to maintain body amino acid homeostasis and function in apparently healthy subjects characteristic of the south Asia region. They further indicate that our previously proposed, tentative lysine requirement of 30 mg.kg-1.d-1 is probably adequate for this population.