Cu L edge X-ray magnetic circular dichroism (XMCD) spectroscopy has been used to study the electronic structure of the blue copper site in plastocyanin. Application of the orbital angular momentum sum rule leads to an experimental (Cu 3d specific) orbital moment [L-z] of 0.07 +/- 0.02 (h) over bar/Cu atom. This compares to 0.054 (h) over bar, derived from the EPR measurements and 0.059 (h) over bar/Cu atom from a SCF-X alpha-SW calculation. Application of the spin angular momentum sum rule leads to an experimental spin moment [S-z] of 0.18 +/-0.02 (h) over bar/Cu atom, compared to 0.21 (h) over bar/Cu atom by SCF-X alpha-SW. XMCD sum rule analysis should be a useful probe of electronic structure for many other problems in inorganic and bioinorganic chemistry.