All of the plasma sulfhydryl (SH) groups are protein associated. Albumin is the most abundant plasma protein (40-60 mg/ml) and is a powerful extracellular antioxidant. Plasma SH groups are susceptible to oxidative damage and are often low in patients suffering from diseases, such as coronary artery disease and rheumatoid arthritis. Additionally, protein SH (P-SH) groups, plasma contains small amounts of glutathione (GSH). Decreased plasma GSH is reported in human immunodeficiency virus (HIV) infection. A spectrophotometric assay based on 2,2-dithiobisnitrobenzoic acid (DTNB or Ellman's reagent) is commonly used for thiol assay. Most of the procedures are for cellular thiols. This chapter describes convenient assays for P-SH and GSH in plasma using spectrophotometric and spectrofluorometric methods. The P-SH level of plasma is calculated by subtracting the GSH level from the T-SH level. There normally is little difference between T-SH and P-SH because of the low GSH levels in the plasma.