A rubber biosynthesis stimulator protein (RBSP) from the C-serum of Hevea brasiliensis latex was purified and characterised. This proteinaceous factor stimulates the incorporation of isopentenyl diphosphate into rubber and thereby enhances the rate of rubber biosynthesis. The heat stable RBSP is shown to have a molecular weight of 13 068 Da by mass spectroscopy. Amino acid sequence of peptide fragments obtained from trypsin digestion revealed regions of sequence similar to eukaryotic Initiation Factor 5A (elF-5A), present in alfalfa (Medicago sativa), tobacco (Nicotiana tabacum and Nicotiana plumbaginifolia), Arabidopsis thaliana and potato (Solanum tuberosum). Three Hevea elF-5A clones were obtained from screening a latex cDNA library with a partial elF-5A cDNA from tobacco. Small variations in their DNA sequences indicated that they represented three different isoforms.