Abstract In the present study, a new flow cytometric method for the identification of TNF-α-secreting cells based on the use of a TNF-α converting enzyme (TACE) inhibitor compound (BB3103) is described. TNF-α secreting cells were measured in parallel in stimulated peripheral blood samples ( n=4), using the BB3103 TACE inhibitor or brefeldin A as secretion blocking agents. To induce TNF-α production by PB T-cells and monocytes, whole blood samples were stimulated either for 4 h with PMA plus ionomycin or for 6 h with LPS plus IFNγ, respectively. Interestingly, slightly higher percentages of TNF-α + CD4 + (65±11% versus 49±11.4%, p=0.06) and TNF-α + CD8 + (60±9.9% versus 47±27.7% p=0.46) T-cells together with a greater amounts of TNF-α/cell—mean fluorescence intensity (MFI) of 1050±230 versus 258±112 for CD4 +, p=0.06 and 424±169 versus 266±201 for CD8 +, p=0.27—were found for activated T-lymphocytes cultured with BB3103 as compared to those treated with brefeldin A. Kinetic analysis of surface TNF-α expression under these stimulatory conditions showed detectable surface TNF-α levels on both T-cells and monocytes after 30 min. Thereafter, surface TNF-α expression on both T-cells and monocytes progressively increased for up to 3 and 4 h, respectively. From this time on, a decrease in the membrane levels of TNF-α was observed in the monocytes, presumably due to the occurrence of cell death. In order to show that the BB3103 inhibitor was also active on other TACE-associated molecules, CD62L expression on PMA-stimulated PB lymphocytes, monocytes and neutrophils was analyzed by flow cytometry in the presence and absence of BB3103. The TACE inhibitor proved to be active in stabilizing CD62L expression on PMA-stimulated PB leukocytes. In summary, our results show that stimulation of PB T-cells and monocytes in the presence of the TACE inhibitor BB3103 followed by surface staining for TNF-α provides a new, simple and rapid method for the identification of intact TNF-α producting cells present in a sample without the need for prior cell fixation and permeabilization. In addition, this approach could also be applied in order to stabilize the expression of other metalloprotease-sensitive molecules such as CD62L on the surface of PB leukocytes.