Plasma free amino acid levels were determined in subjects suffering from pellagra and compared with levels observed in normal subjects of both low and high socioeconomic groups. In pellagrins, a wide variation was seen in the plasma levels of tryptophan and these levels overlapped considerably with those in the low socioeconomic group controls. Administration of 5 g L-leucine daily for 5 days to normal subjects belonging to the low socioeconomic group did not affect the plasma tryptophan levels. These data suggest that the pellagragenic action of leucine is not mediated through changes in circulating levels of tryptophan. They also indicate that a low level of plasma tryptophan per se may not be a reliable biochemical indicator in the diagnosis of pellagra.