AbstractIndole‐3‐acetic acid (IAA) contents in plant extracts were determined spectrofluorometrically after conversion of the compound into indolo‐α‐pyrone. The method is sensitive and accurate and showed good reproducibility. The values obtained by this method corresponded well with those obtained with the Avena coleoptile straight‐growth assay.Impurities in extracts often cause various difficulties. Colored compounds influence the readings and may even make indole‐3‐acetic acid determination impossible by strong quenching of the fluorescence light. Insoluble compounds can be formed in the reaction and cause increased opalescence and light scattering which interferes with the fluorescence measurements. The necessities for the method to be reliable are that the insoluble compounds are removed or brought into solution and that the exciting light is sufficiently monochromatic.Indolo‐α‐pyrone is fairly stable in the dark but is rapidly broken down by UV radiation. This decay may be used as a check to see if the measured light represents fluorescence