The parameter estimation of a large-scale interconnected static linear system is treated with the structure described by an interconnection matrix. The standard least-squares algorithm is investigated, and its ability to recover, with probability one. composite system parameters from the data accessible in the complex system is examined. The limit properties of the least-squares estimate in the particular context of a complex system are derived, and both local (referring to individual system elements) and structural (relating to the complex system in the large) necessary and sufficient applicability conditions are established.