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Effects of low temperature on rubber particle proteins and rubber transferase activity were investigated in stem bark tissue of guayule (Parthenium argentatum, cv. 11591), a desert shrub of commercial interest as an additional source of natural rubber. Three-year-old guayule plants were subjected to 60 cycles of 15/10 degrees C day/night temperature regime with a 12-h photoperiod in a controlled environment growth chamber. Rubber particles were extracted and the associated proteins were characterized in stem tissues of guayule. SDS-PAGE protein profile of the rubber particles showed an abundant rubber particle protein (RPP) with a molecular mass of 50 kDa accumulating at higher concentrations under low temperatures. The higher levels of this polypeptide persisted for 60 cycles when the low temperature stress was continued in the growth chamber. Low temperatures also induced rubber transferase activity, estimated from the rubber particle fractions. The correlation between rubber production and net photosynthetic rates was examined in low temperature-treated guayule. There was a two-fold increase in rubber content associated with a high photosynthetic rate. The results indicate that rubber biosynthesis in plants treated with 15/10 degrees C low temperature for 60 cycles was superior to that of those grown under natural photoperiod and the increased activity of rubber transferase could possibly result from enhanced RPP synthesis. Further characterization of this polypeptide will be vital for understanding why enhanced rubber production is restricted to the winter months in field-grown guayule.