Parthenium argentatum is a prime candidate for development as a new crop for theproduction of natural rubber (cis-1,4-polyisoprene). The rubber obtained from P. argentatum iscomparable in molecular weight (and quality) to that obtained from Hevea brasiliensis (Mr ca.106 Da), which is currently the sole commercial source of natural rubber. Most of the 2500rubber-producing plant species known make much lower molecular weight rubber, although theregulation of polymer molecular weight is not understood. In the experiments reported here,we investigated the regulation of rubber biosynthesis and polymer molecular weight usingpurified, enzymatically-active rubber particles isolated from P. argentatum, under variousconcentrations of the allylic diphosphate (allylic-PP) rubber molecule initiators farnesyldiphosphate (FPP, a 15-carbon molecule), or geranyl diphosphate (GPP, a 10-carbon molecule)and the elongation substrate isopentenyl diphosphate (IPP). Our results show that the rates ofboth rubber molecule initiation and polymerization, and the final polymer molecular weight,were greatly affected by the concentration of initiator and IPP. Increasing allylic-PP initiatorconcentrations caused an increase in the amount of rubber synthesized and the number ofmolecules initiated but a decrease in the mean polymer molecular weight; increasing IPPconcentrations increased the amount of rubber and the mean polymer molecular weight. Atidentical substrate concentrations of IPP and allylic-PP, GPP initiated about one third of therubber polymers initiated by FPP but incorporated about two thirds the amount of IPP, comparedto FPP. Consequently, while the amount of rubber synthesized in the presence of GPP was lessthan with FPP, the rubber polymers synthesized with GPP initiator were, on average, about twicethe molecular weight of those with FPP.