AbstractA range of 18 derivatives of flavone‐8‐acetic acid (FAA) with substituents on the 2‐phenyl group have been prepared and their anti‐tumour activity evaluatedin vitroagainst a panel of human and murine tumour cell lines andin vivoagainst MAC 15A. There was no clear‐cut relationship betweenin vitroandin vivoactivity but the activity in each situation was found to be very sensitive to the precise substitution pattern with closely related isomers giving widely different activities. Some of the compounds, notably10b,cj, andr, were activein vivoand these require further studies in order to evaluate their potential for de