The electronic structure of the blue-copper site of Pseudomonas aeruginosa azurin has been investigated by ab initio multireference determinantal configuration interaction (MRD-CI) calculations, A truncated site consisting of copper and its three equatorial ligands has been studied with emphasis on the g tensor and the nitrogen hyperfine tensors of the coordinating histidines. In the ground state the singly occupied molecular orbital (SOMO) involves a copper 3d orbital pi antibonded to the cysteine sulfur and c antibonded to the histidine nitrogens. A proper description of the electron-paramagnetic-resonance parameters has been achieved through the use of an effective core potential for copper up to and including the 3s electrons. Both the complete g tensor and the anisotropic hyperfine tensors at the nitrogens are essentially reproduced. Mulliken spin densities of 35 and 59% on copper and sulfur, respectively, and 2.1 and 1.7% on the respective coordinating nitrogens reflect the delocalized character of the SOMO and the inequivalence of the histidines.