Gibberellins stimulate the release of reducing sugars and hydrolytic enzymes from barley endosperm (Paleg, 1960; Yomo, 1960; McLeod etal., 1962; Jones and Vabneb, 1967). The release of reducing sugars, which was shown to be proportional to the logarithm of applied gibberel lin concentration, has been used as a bioassay for gibberellins (Coombe et al., 1966). Vabneb and his co-workers have found the release of a amylase by isolated barley aleurone layers to be a more convenient method of bioassay (Jones and Vabneb, 1967). Both methods have been well documented and extensively used. In the course of a study on the gibberellin induced release of hydro lytic enzymes by barley endosperm, it was found that treatment with gibberellic acid induced the production of substantial quantities of peroxidase. The ease of measurement of peroxidase activity led us to investigate the sensitivity of production of this enzyme to added gibbe rellic acid, with a view to using the response as a bioassay for gibberellins. Barley seeds, var. Beorna were dehusked in 50% sulphuric acid, halved, and the endosperm halves hydrated at 5° C for 20 hours. Twenty half seeds were set up in a crystallising basin containing 5 ml of test solution. Smaller seed numbers and volumes of test solution may be used without any decrease in sensitivity. The seed halves in test solution were incubated at 30° C for 24 hours, after wich the ambient medium was collected and the seeds discarded or retained if needed for electrophoretic studies. The peroxidase activity of the medium was then assayed by the O-dianisidine method (Maehly and Chance, 1954). All operations were carried out under sterile conditions.