  We propose a directly measurable criterion for the entanglement of two
qubits. We compare the criterion with other criteria, and we find that for pure
states, and some mixed states, it coincides with the state's concurrency. The
measure can be obtained with a Bell state analyser and the ability to make
general local unitary transformations. However, the procedure fails to measure
the entanglement of a general mixed two-qubit state.
