Post-prandial (p.p.) changes in plasma free amino acid (AA) concentrations of children consuming a single source of protein at critical levels are determined by its digestibility and total essential AA/total AA ratios; the molar proportion of the limiting EAA (EAA/TEAA), if any, will fall significantly in plasma as it is utilized more completely than others. Grain amaranths (Am), reputedly rich in lysine (Lys) and tryptophan (Trp), but moderately deficient in leucine (Leu), should be ideal complements to Lys and Trp-poor, Leu-rich maize (M). Most animal studies confirm this. In children, 20, 30 and even 50 per cent replacement of M proteins with toasted Am proteins had failed to show any gains over M or Am alone: heat losses of Lys were suspected. Plasma obtained during the above studies, before and 3 h and 4 h after the first meal of the last day of consuming Am alone, three M-toasted Am mixtures, or M alone, were analysed for free AAs. Toasted, popped or flaked Am consumption caused significant p.p. falls in molar proportions of Leu from 99 to 85, 88 to 82, and 92 to 75, and of threonine (Thr) from 118 to 108 (popped) and 109 to 97 (flaked) mmol/mol TEAA, suggesting that these were first- and second-limiting EAAs. Post-prandial fall in Lys proportion was questionably significant. The M diet produced highly significant 3-h Leu elevation from 132 to 187, Lys fall from 167 to 135, and Trp fall from 62 to 46 mmol/mol TEAA.(ABSTRACT TRUNCATED AT 250 WORDS)