The relationship between greening and production of flavour compounds was investigated in blanched and green young (10-week) and mature (20-week) celery plants, and in colourless and green celery cell suspensions. In the young plants the concentrations of limonene and other terpenoids increased in the petiole and leaf-extract as the level of chlorophyll increased. In the cell cultures, greening was induced by transferring cells to a medium where 2,4-dichlorophenoxyacetic acid (2,4-D) was replaced by 3,5-dichlorophenoxyacetic acid (3,5-D). During the first subculture, phthalides were produced in the growth phase, but were present in trace amounts only in the second and third subculture. Limonene, however, increased after the first subculture and during the growth phase of the third subculture was comparable to the levels in the young celery plant. Greening and aggregation of the cells had also increased by the third subculture in the 3,5-D-containing medium. It was suggested that phthalide production was stimulated by the transfer from a 2,4-D- to a 3,5-D-containing medium, but as the level of aggregation and greening increased in the culture, phthalide production was reduced and limonene production increased. Electron and light microscope pictures showed that, despite the greening and aggregation, there was no chloroplast formation or cell differentiation in the cultures incubated with 3,5-D. The role of greening in stimulating the formation of celery flavour compounds in tissue cultures is discussed.