The biological activities of gibberellin A9 (GA9), gibberellin A12 (GA12) and monofluoro-analogues (F-GA9 and F-GA12), substituted in the 1 β-methyl group, were compared in the barley endosperm, cucumber hypocotyl, lettuce hypocotyl, 'Meteor' dwarf pea, dwarf-5 maize and Rumex leaf disc assays. In most cases the fluorosubstituted compounds had a potency similar to, or less than, the relevant unmodified gibberellin but, in the lettuce assay, F-GA9 was approximately 5 times more active than GA9 up to a dose rate of 10(-1) μg.A 27-30% mixture of fluorogibberellin A3 (F-GA3) in GA3 had a lower activity than 100% GA3 in the barley endosperm, lettuce hypocotyl and dwarf maize assays. This suggested that pure F-GA3 may be a competitive inhibitor of GA3 action. The findings are discussed in the context of the structure/activity relationships of the gibberellins.