Lysine and S(2-aminoethyl)cysteine (AEC) metabolism were investigated in normal barley (Hordeum vulgare L. cv. Bomi) and a hemozygous recessive AEC-resistant mutant (R906). Feedback regulation of lysine and threonine synthesis from [(14)C] acetate was unimpaired in plants of the mutant 3 d after germination. Seeds of Bomi and R906 contained similar total amounts of lysine, threonine, methionine and isoleucine. Concentrations of these amino acids in the soluble fraction of plants grown 6 d without AEC were also similar. The concentration of AEC in R906 plants was less than in the parent variety when both were grown in the presence of 0.25 mM AEC for 6 d. The uptake of [(3)H]AEC and [(3)H]lysine by roots of R906 was, respectively, 33% and 32% of that by Bomi roots whereas the uptake of these compounds into the scutellum was the same in both the mutant and its parent. The uptake of [(3)H]leucine and its incorporation into proteins was also the same in Bomi and R906 plants. These results suggest that a transport system specific for lysine and AEC but not leucine is altered or lost in roots of the mutant R906. AEC is incorporated into protein and this could be the reason for inhibition of growth rather than action as a false-feedback inhibitor of lysine biosynthesis.