The invention relates to measuring immunological tolerance in T cells. In particular, the invention provides a method for measuring the tolerance status of a T cell sample, comprising the steps of (a) measuring the expression level of XCR1 (also known as GPR5, which is a receptor for Single C Motif-1/Lymphotactin) in a T cell sample, and (b) comparing the measured expression level with a reference value, said reference value being representative of the expression level of XCR1 in a control T cell sample. The present invention furthermore provides methods, kits and uses to determine the required stringency of immunosuppression, particularly the required dosage of an immunosuppressant. Also provided are methods and uses to identify or validate inducers or inhibitors of immunological tolerance.