Lysine demethylases containing Jumonji C (JmjC) domains produce formaldehyde following 2-oxoglutarate-dependent demethylation. Cayman's Demethylase (jumonji-type) Activity Assay provides a convenient fluorescence-based method for assaying JmjC-mediated demethylase activity from cell lysates or purified enzyme preparations. The assay is based on the production of formaldehyde during the demethylation of a methylated peptide substrate. Cyclization of formaldehyde and acetoacetanilide in the presence of ammonia gives a fluorescent product for quantitation.