The crude extract prepared from Glycyrrhiza echinata cells treated with yeast extract catalyzed the formation of liquiritigenin (5-deoxyflavanone) and isoliquiritigenin (6'-deoxychalcone) in addition to naringenin (5-hydroxyflavanone) when incubated with 4-coumaroyl-CoA and malonyl-CoA in the presence of high concentrations (0.1 mM or higher) of NADPH. Incubation without NADPH, or with low concentrations (0.01 mM or lower), gave only naringenin as a reaction product. With NADH (1 mM), the major product was naringenin accompanied by a small quantity of liquiritigenin. The initial product of the assay with 1 mM NADPH was isoliquiritigenin, indicating a reaction catalyzed by 6'-deoxychalcone synthase (DOCS). Subsequent formation of liquiritigenin was attributed to the presence of chalcone isomerase in the crude extract. The results constitute the first demonstration in vitro of DOCS activity which, in G. echinata cells and other leguminous plants, is involved in the biosynthesis of retrochalcone and 5-deoxyisoflavonoid-derived phytoalexins.