Biosynthetic studies of the guaianolide-type sesquiterpene lactones 11βH,13-dihydrolactucin-8-O-acetate and 8-desoxylactucin were performed in Agrobacterium rhizogenes--transformed hairy root cultures of blue-flowered lettuce, Lactuca floridana. The 1 3 CNMR spectra of the two guaianolides labelled by incorporation of [1- 1 3 C], [2- 1 3 C], [1,2- 1 3 C 2 ]acetate and [2- 1 3 C]mevalolactone showed patterns of enrichment consistent with a previously proposed biogenetic pathway for guaianolide-type sesquiterpene lactones via the acetate-mevalonate-germacradiene route.