Recent studies have implicated reduced thiols (cysteine −SH) in the function of individual cell surface proteins. Studies presented here demonstrate that the overall level of reduced thiols on cell surface molecules differs on individual subsets of peripheral blood mononuclear cells and that these levels can be manipulated in vitro by altering the level of intracellular glutathione (iGSH). To quantitate cell surface thiols, we have developed a Hi-D (11-color) fluorescence-activated cell sorter method in which we covalently couple a fluorescent molecule, Alexa-maleimide, to free (reduced) –SH groups on proteins or other molecules exposed on the cell surface (exofacial membrane). In addition, to reveal changes in cell surface thiol levels in response to various in vitro treatments, we used a pair of fluorescent Alexa dyes with distinct excitation and emission spectra to stain the cells before and after treatments. These in vitro studies demonstrate that decreasing iGSH, by specifically inhibiting its synthesis, decreases cell surface molecule thiols (csm−SH) and that preventing loss of iGSH also prevents loss of csm−SH. However, examination of peripheral blood mononuclear cell subsets tested immediately after isolation from healthy or HIV-infected subjects failed to reveal a similar relationship between internal iGSH and csm−SH. Although there is a relatively wide variation between individuals in both csm−SH and iGSH, there is no correlation between median iGSH and csm−SH compared for 22 healthy and 36 HIV-infected subjects. Collectively, our findings indicate that local environment plays a greater role in determining the redox status of cell surface molecules than the internal redox status of the cells.