ALTHOUGH extinction coefficient data are available for 6-furfuryl-amino purine (kinetin)1 it is not always possible to use these in its estimation. During investigations of the metabolism of kinetin in plant tissue we have found that interference by ultra-violet absorbing contaminants prevents the direct spectrophotometric assay of this compound in the plant extracts. The following colorimetric method, based on a procedure described by Bratton and Marshall2, has now been developed and found to give satisfactory results.