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Heterotrophic cell suspension cultures were initiated from hypocotyls of young Ammi majus L. seedlings. When these cultures were propagated continuously in the dark, they produced only traces of the coumarin umbelliferone. Upon addition of fungal cell wall fractions, i.e. a skieroglucan or an elicitor from either Phytophthora megasperma f. sp. glycinea or Alternaria carthami , the cells excreted large amounts of umbelliferone in addition to isopimpinellin, ( S )-marmesin, ( R )-ammirin, umbelliferone-[3′-methyl-buta-1 t .3-dien-1′-yl]-ether and umbelliferone-[3′-hydroxymethyl- 1 t .-buten-1′-yl]-ether. The last two compounds are new compounds and appear to be derived from 7- O -prenylumbelliferone, a coumarin that has been identified in from other plants. The Phytophthora -elicitor was the most effective inducer of coumarin accumulation. When cultures which had been elicited for 3 hr were pulsed with l -[U- 14 C]phenylalanine for 7 hr, all the coumarins and an additional, still unidentified compound in the extracts were labelled efficiently. Our results suggest that A. majus cells are particularly suitable for biosynthetic studies on various coumarins as well as for regulatory studies of the inhibition of coumarin phytoalexin accumulation.