The effect of six unique isoforms of Hevea brasiliensis eukaryotic translation initiation factor 5A (eIF-5A), previously reported as a rubber biosynthesis stimulator protein (RBSP) was evaluated using biosynthesis assays containing zone 2 rubber particles and radiolabelled isopentenyl diphosphate ([14C]-IPP). Recombinant RBSP proteins were over-expressed as N-terminal glutathione-S-transferase (GST)-tagged proteins in a bacterial system. Crude extracts of six RBSP-GST isoforms were added to rubber biosynthesis assays in increasing volumes. Results showed that increases in the amount of each RBSP-GST isoform corresponded with increases in the amount of each RBSP-GST isoform corresponded with increases in the level of [14C]-IPP incorporation above that of the control which contained no recombinant protein. Statistical data analysis revealed that three RBSP isoforms increased [14C]-IPP incorporation significantly compared to the GST control where the RBSP isoform with the highest stimulatory effect produced a 45;increase in IPP incorporation compared to GST.