Thiol groups in living organisms primarily comprise non-protein thiols and protein thiols. Thiol compounds possess important detoxification functions and hold significant physiological relevance for the self-regulation of organisms. This assay kit provides a convenient method for detecting non-protein thiols in biological samples, such as animal and plant tissues, serum, or plasma. In this kit, thiol groups react with 5,5’-dithio-bis-(2-nitrobenzoic acid) (DTNB), generating a yellow-colored compound which exhibits a maximum absorption peak at 412 nm.