Spinach chloroplast thioredoxin f has a third cysteine residue which is surface exposed and close to the active site disulfide. In addition its N-terminus is rather long compared to other thioredoxins. By site-directed mutagenesis the third cysteine has been replaced, the long N-terminal tail has been removed and the properties of the modified proteins have been examined. Truncation of the N-terminus renders the protein more soluble and stable and has little influence on its catalytic capacities. Replacement of the exposed third cysteine clearly impairs its capacity to interact and reduce target enzymes and shows that this cysteine can be involved in homo-dimer formation.