Sieve tubes play important roles in the transfer of nutrients as well as signals. Hundreds of proteins were found in pure phloem sap collected from rice (Oryza sativa L. cv. Kantou) plants through the cut ends of insect stylets. These proteins may be involved in nutrient transfer and signal transduction. To characterize the nature of these proteins, the partial amino-acid sequence of a 13-kDa protein, named RPP13-1, that was abundant in the pure phleom sap was determined. A cDNA clone of 687 bp, containing an open reading frame of 122 amino acids, was isolated using corresponding oligonucleotides as a probe. The deduced amino-acid sequence was very similar to that of the ubiquitous thiol redox protein, thioredoxin. The consensus sequences of thioredoxins are highly conserved. No putative signal peptide was identified. Antiserum against wheat thioredoxin h cross-reacted with RPP13-1 in the phloem sap of rice plants. RPP13-1 produced in Escherichia coli was reactive to antiserum against wheat thioredoxin h. Both E. coli-produced RPP13-1 and the phloem sap proteins catalyzed the reduction of the disulfide bonds of insulin in the presence of dithiothreitol. These results indicate that an active thioredoxin is a major protein translocating in rice sieve tubes.