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The detection of ferromagnetic Ni nanowires with integrated giant magnetoresistance sensors is investigated for possible use in magnetic biosensing. Single nanowires with magnetic moments at least as small as 1.5×10-10 emu are readily detectable in a zero-applied field. The signal is a linear function of the number of nanowires, making the sensor suitable for use as a particle counter. With increasing wire length, the magnitude of the sensor signal increases at first and then appears to saturate. The signal shows significant dependence on the angle of the wire with respect to the sensing axis and changes sign when the wire's moment is antiparallel to the sensing direction.