Whether 3-epigibberellin(3-epiGA 1 ) is endogenous in plants or is formed from GA 1 as an artefact, can be determined by adding [ 1 3 C]GA 1 to the plant material as an internal reference and measuring the 1 2 C: 1 3 C ratio of 3-epiGA 1 and GA 1 , recovered from the plant extract. We use this method to show that 3-epiGA 1 is not a natural constituent of vegetative tissues of Zea mays (maize) but is endogenous in Lactuca sativa (lettuce).