Rubber transferase (RT) from Hevea brasiliensis was found to be a membrane protein located on rubber particles and not a hydrophilic protein existing in latex serum. The activities of RTs in rubber trees from cultivars SCATC4, PR107 and SCATC35 which have been tapped for several years were determined under different temperatures ranging from 4 C to 50 C. The results show the highest activity of RT at 25 C and a positive correlation at any temperature between the RT activities and the yields of various cultivars. Detection o RT activities of one year old trees from SCATC7-33-97, RRIM600, Haikenl, PB86 and Tianren 3145 demonstrates a significant positive correlation between their RT activities and rubber yields. These facts suggest that RT should be key enzyme in rubber biosynthesis and hence the possibility of creating high yielding and resistant rubber trees by RT cloning and transfering. RT is likely to become a reliable index for yield prediction of rubber trees at the nurserny stage in breeding programme.