Sporophores from five species of Lactarius mushrooms had a liquid rubber content of 0.1% to 7% based on the dry weight. Rubber from L. volemus, L. chrysorrheus and L. hygrophoroides was found to be a homologue of polyprenol being composed of dimethylallyl group, two trans isoprene units, 160-300 cis isoprene units, and terminal hydroxyl or ester group aligned in that order by 13C-NMR analysis. The ratio of fatty acid ester group to hydroxyl group was about 9/1 to 5/5. The number of both terminal groups and trans units decreased during aging of sporophores. Rubber from L. piperatus, L. vellereus and L. subpiperatus was found to be cis polyisoprene having very small quantities of both terminal groups and trans units. The biosynthesis of cis polyisoprene in Lactarius mushrooms was found to start from trans, trans-farnesyl pyrophosphate. The termination was assumed to occur by esterification of polyisoprenyl pyrophosphate. Occurrence of some chemical modifications on both terminal groups was presumed during aging of sporophores.