The primary structure of thioredoxin f from spinach chloroplasts was determined by standard amino acid sequencing and furthermore by sequencing the corresponding nuclear genome region. The protein, with a calculated molecular mass of 12,564 Da and a molar absorption coefficient at 280 nm of 17,700 M-1 cm-1, consists of 113 residues and exhibits 24% residue identities with spinach chloroplast thioredoxin mb or Escherichia coli thioredoxin. A monospecific antibody elicited against thioredoxin f has been used to select recombinant phage from spinach cDNA libraries in lambda gt11. The inserts of positive clones were sequenced. They code for a polypeptide of 190 amino acids, composed of the thioredoxin f sequence (113 residues) and an upstream element (77 residues) which most probably constitutes the N-terminal transit peptide that directs the polypeptide into chloroplasts. In vitro transcription and translation of this construct generates a polypeptide of approximately 21 kDa, which is imported by isolated spinach chloroplasts and processed to the mature 12.5-kDa protein.