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Detection and isolation of viable alloreactive T cells at the single-cell level requires a cell surface marker induced specifically upon T cell receptor activation. In this study, a member of the tumour necrosis factor receptor (TNFR)-family, CD137 (4–1BB) was investigated for its potential to identify the total pool of circulating alloreactive T cells. Optimal conditions for sensitive and specific detection of allogeneic-induced CD137 expression on circulating T cells were established. Thereafter, CD137+ alloreactive T cells were phenotypically and functionally characterized by multi-parameter flow cytometry. Alloantigen-induced CD137 expression identified both alloreactive CD8+ T cells (mean ± standard error of the mean: 0·21 ± 0·07%) and alloreactive CD4+ T cells (0·21 ± 0·05%). CD137+ alloreactive T cells were detected in different T cell subsets, including naive T cells, but were found preferentially in CD28+ T cells and not in the terminally differentiated T cell subset. Upon allogeneic (re-)stimulation, the cytokine-producing as well as proliferative capacity of T cells resided mainly within the CD137-expressing fraction. About 10% of the CD137+ alloreactive T cells produced any combination of interferon (IFN)-γ, interleukin (IL)-2 and TNF-α. Polyfunctional alloreactive T cells, defined by multiple cytokine expression, were observed infrequently. In conclusion, activation-induced CD137 expression is a fast assay allowing for detection and functional analysis of the total alloreactive T cell compartment at the single-cell level by multi-parameter flow cytometry.