The proteindisulfide isomerase (PDI), a multifunctional cytoplasmic enzyme with additional chaperone activity, has been shown recently, using monoclonal antibodies, to be located on the membrane of mature human B lymphocytes and B cell chronic lymphocytic leukemia (B-CLL) cells. Here, evidence is presented that this antigen exhibits catalytic activity as measured by the reductive degradation of insulin (release of A chain molecules) on intact B cells in patients suffering from B-CLL, as well as on JVM 13 cells (B-CLL cell line). More than 98% of these cells exhibited PDI activity which could be inhibited by bacitracin and also by monoclonal and polyclonal antibodies to PDI. Interestingly, surface PDI expression was strongly correlated in our study with protein-bound membrane SH groups. These surface protein thiols were specifically determined by using low concentrations of the chloromethyl-derivative based fluorescent probe 5-(and6)-(((4-chloromethyl)-benzoyl)amino)-tetramethyl-rhodamine (CMTMR) at low temperature in the presence of sodium azide in flow cytometry. The highest PDI and SH expression was found on B lymphocytes, particularly B-CLL cells. The mean fluorescence intensity (MFI) of CMTMR-positive B cells in the B-CLL line was up to 10-fold higher than that of controls, indicating a strong elevation of cell membrane-located protein thiols on malignant B cells. The link between PDI and SH expression on cell surfaces points to a functional interaction between the two. Treatment with bacitracin resulted in a strong inhibition of PDI and a dramatic increase in surface protein thiol expression of B-CLL cells. Similar effects could be observed by cell treatment with anti-PDI antibodies, indicating that this enzyme system plays a crucial role in the regulation of protein-bound SH groups. Interestingly, artificially induced protein thiol expression led to significantly higher cellular resistance to the cytostatic drugs chlorambucil, vinblastin, and cisplatin in vitro as measured by cell growth. These data suggest for the first time a regulatory effect of PDI on the surface protein thiol status of B cells. The increased expression of PDI may play a crucial role in SH-mediated protection and drug resistance in malignant B lymphocytes.