The intense colour of anthocyanins has made it possible to identify the individual cells accumulating these compounds within cultures of Catharanthus roseus. Furthermore, the amount of anthocyanin per cell could be measured directly by microdensitometry thus revealing the degree of heterogeneity within the cell populations at any interval during the growth cycle. In this way the contribution to overall product yield of individual cells in the culture was assessed. It has been found using this approach that although some inter-experiment variation occurs, intracellular anthocyanin concentration varies very little and is therefore likely to be controlled by a feedback inhibition mechanism. In general, attempts to enhance the mean intracellular concentration of the pigment through the manipulation of culture conditions have failed. A possible mechanism of metabolic control of intracellular anthocyanin accumulation is discussed. The proportion of cells accumulating anthocyanins, however, varied considerably in cultures grown under different conditions but nevertheless always remained low, never exceeding 20% of the total cell population. The cause of this poor response is unclear but it is suggested that a close metabolic interaction may exist between the cells in the culture to bring about this effect. The possible relevance of these observations to other culture systems accumulating secondary metabolites is discussed.