SUMMARYOnion callus was exposed to high concentrations of a number of intermediates involved in the synthesis of S-fraw5-prop-l-enyl-L-cysteine sulphoxide, the major flavour precursor in the intact onion plant. The intermediates were sodium sulphate, L-valine, methacr\'lic acid and S-(2-carboxypropyl)-L-cysteine. Growth was reduced or totally inhibited at the highest concentration (50 to 100 mM) of all the intermediates but, except for methacrylic acid which was inhibitory at all concentrations, a limited growth occurred at the lower concentrations (10 to 20 HIM) of the other compounds. The only treatment which caused the callus to produce an onion odour on crushing the tissue was when S-(2-carboxypropyl)-L-cysteine was incorporated in the culture medium; this result indicated that the callus was capable of the final stages in the synthesis of S-fr<2W5-prop-l-enyl-L-cysteine sulphoxide. The presence of this pathway was confirmed by feeding smaller amounts of L-cysteine, L-valine and S-(2-carboxy-propyl)-Lcysteine (1 and 3 mM) and separating the amino acids and flavour precursors by t.l.c. electrophoresis. 5-^ram-prop-l -enyl-L-cysteine sulphoxide was found only when S-(2-carboxypropyl)-L-cysteine was present in the medium. When S-ethyl and S-propyl-L-cysteine were fed to the callus these compounds were oxidized to their sulphoxides showing that an oxidase system for the S-alkyl cysteines was present in the callus.