Two cell lines of Tabernaemontana divaricata derived from the same suspension culture accumulate different amounts of the terpenoid indole alkaloids O-acetylvallesamine and voaphylline. [(15)N]O-acetylvallesamine and [(15)N]voaphylline were added to the suspension cultures to investigate whether the lack of accumulating capacity of one of the cell lines was due to a low biosynthetic ability or to high turnover rates. The difference was shown to be due to the inability of the cell culture to biosynthesize both alkaloids. Both cell lines were able to metabolize O-acetylvallesamine. This metabolisation occurred mainly during the stationary phase. The alkaloids added were chemically unstable under culture conditions. Under normal batch cell culture conditions chemical breakdown is thought to play a minor role in the total amount of compound transformed.