A highly sensitive method for the assay of catechol-O-methyltransferase in erythrocytes is described, which employs high-performance liquid chromatography with fluorescence detection. A newly synthesized catechol compound, 2-(3,4-dihydroxyphenyl)naphtho [1,2-d]thiazole is used as a highly fluorogenic substrate for catechol-O-methyltransferase; the m- and p-methylated products formed enzymatically from the substrate under the optimum conditions, after extraction with n-hexane--chloroform, are separated by normal-phase chromatography on LiChrosorb Si 100. The limits of detection for m- and p-methylated products are 3 pmol per assay tube (60 fmol per injection volume of 20 microliter) in each case. The ratio of m- and p-methylated products was 0.54. This method requires as little as 50 microliter of human erythrocytes.