In this thesis a microfluidic device for manipulating and detecting magnetic particles (MPs) suspended in a microfluidic channel is presented. Current carrying conductors are used, to move the MPs to a giant magnetoresistance (GMR) sensor, where they are detected. MPs can be functionalised by modifying their surface, to enable them to chemically bind to a biological cell. Thus the device could be integrated in a lab-on-a-chip (LOC), to indirectly detect biological cells. Calculations concerning the magnetic field of the conductors and the magnetic field of the MPs have been performed to determine the optimal position of the GMR sensor and estimate the output voltage of the system. Two different chip designs have been fabricated. One with a small GMR sensor to detect single MPs and another one, which attracts MPs from a wide area to a bigger GMR sensor, to detect low concentrations of MPs. Several experiments, in which different MPs have been successfully detected, have been conducted as a proof of concept.