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A method is introduced for the quantitative determination of the interference level, originated by a direct-sequence (DS) spread-spectrum electromagnetic signal, impinging a two-wire transmission line (TL) on which a similar information signal is driven by a lumped voltage source. The model presented makes use of a properly modified, second-order accuracy finite-difference time-domain (FDTD) algorithm, the so-called complex-envelope (CE) FDTD, incorporating the principle concept of the DS code division multiplex access method. Moreover, this paper examines the advantages arising from the use of the CE representation of the final FDTD expressions, allowing for an accurate solution of the electromagnetic interference (EMI) problem by partially avoiding the computational burden of the classic FDTD approach. Numerical results for several general cases are presented, providing valuable information for the comprehension of the influence of the amplitude and angle of incidence of the interfering signal to the TL behavior.