This paper is concerned with a direct-sequence codedivision multiple-access (DS-CDMA) system operating over a Rayleigh fading and sharing common spectrum with a narrowband waveform. A suppression filter at a receiver is employed to reduce the narrow-band interference. We evaluate the average up-link bit error rate (BER) performance and investigate how the performance is influenced by various parameters, such as the number of taps of the suppression filter, the number of multipleaccess users, the ratio of narrow-band interference bandwidth to the spread-spectrum bandwidth, the interference power to signal power ratio, the ratio of the offset of the interference carrier frequency from the spread-spectrum carrier frequency to the half spread-spectrum signal bandwidth, and so on.