Natural rubber was identified for the first time in the latex of Ficus benghalensis, and the rubber biosynthetic activity in latex and rubber particles was investigated. 13C NMR analysis of samples prepared by successive extractions with acetone and benzene confirmed that the benzene-solub el residues were natural rubber, cis-1, 4-polyisoprene. The rubber content in the latex of F. benghalensis was approximately 17;. Gel permeation chromatography revealed that hte molecular mass of the nattural rubber from F. benghalensis was approximately 1 500 kDa. The high rubber content and large molecular size suggest that F.benghalensis is a good candidate for an alternative rubber source. Examination of latex serum from F. benghalensis by SDS-polyacrylamide gel electrophoresis revealed a small number of proteins with major proteins of 31 and 55 kDa in size. The 31-kDa protein was predominant in catalytically-active rubber particles. Determination of metal ion concentration in latex and a comparison of the effect of ethylenediamide-tetraacetic acid on in vitro rubber biosynthesis in F. benghalensis, F. carica and Hevea brasiliensis suggest that the devalent metal ion present in latex serum is an important physiological factor controlling the rubber biosynthetic activities in these plant species. Microscopic examination revealed that the rubber in F. benghalensis occurred in a series of laticifer cells located in concentric zones in the inner bark of stems and branches.