Lysine demethylases containing amine oxidase domains, including the lysine-specific demethylases LSD1 and LSD2, specifically demethylate histone H3 at lysine 4 in a FAD-dependent reaction. Cayman's Demethylase (LSD-type) Activity Assay provides a convenient fluorescence-based method for assaying LSD-type demethylase activity from cell lysates or purified enzyme preparations. The assay is based on the production of formaldehyde during demethylation of lysine 4 on a peptide corresponding to the first 21 amino acids of the N-terminal tail of histone H3. Cyclization of formaldehyde and acetoacetanilide in the presence of ammonia gives a fluorescent product for quantitation.