Human serum albumin (HSA) is a mixture of mercaptalbumin (HMA, reduced form) and nonmercaptalbumin (HNA, oxidized form), i.e., a protein-thiol redox couple in the extracellular fluid (ECF), and it might have antioxidant properties. Forty-two patients with orthopedic disorders participated in this study and were divided into two groups according to their age (young and older groups). By using HPLC to separate HSA into HMA and HNA, we analyzed the percentages of HMA and HNA in serum and lumbar cerebrospinal fluid (CSF). We also examined the redox activity of cultured normal human astrocytes, aortic endothelial cells, and dermal fibroblasts for HSA-thiol. The mean HMA value from the serum of the older group was significantly lower than that of the young group, whereas that from CSF was not significantly different between the two groups; CSF albumin is almost completely in the reduced form, and no age-related differences were observed. Cultured astrocytes and aortic endothelial cells showed conversion of HNA to HMA, whereas dermal fibroblasts showed no such redox activity. From the results obtained from in-vivo and in-vitro studies, HMA is considered to participate in redox regulation in the ECF, for example in the CSF that surrounds the central nervous system (CNS), and in blood serum.