THE incorporation of DL-3-hydroxy-3-methyl-[2-14C] pentano-5-lactone (DL-[2-14C] mevalonic acid lactone) (MVA) into cholesterol in rat-liver homogenates was first demonstrated by Tavormina, Gibbs and Huff1. Subsequent work showing that this lactone is incorporated into squalene2 and β-carotene3 supported the view that mevalonic acid, or a derivative containing the same branched carbon atom structure, is directly involved in the biosynthesis of a wide range of polyisoprenoid compounds. Park and Bonner4 showed that when MVA is incubated with freshly tapped Hevea latex it is incorporated into polyisoprene, although the reported efficiency of conversion was only about 2 per cent. Gascoigne and Jones5 however were unable to observe the incorporation, in vitro, of MVA into rubber with the aid of fresh latex. We now wish to report an investigation which fully confirms the conclusions of Park and Bonner.