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We propose and demonstrate an active contour technique to track fast-rolling leukocytes observed in vivo from video microscopy. A rolling leukocyte is an activated white blood cell that interacts with the vessel wall (the endothelium) in the inflammatory process. Tracking is enhanced here to accommodate fast-moving cells. To tackle the task of tracking wherein only low temporal resolution is possible, we have introduced an energy-minimizing framework and obtained a partial differential equation (PDE) based active contour evolution technique. The proposed PDEs are shown to be an initialization-insensitive version of the gradient vector flow (GVF) proposed by Xu and Prince (1998). We modify the GVF-PDEs by adding a Dirichlet type boundary condition (BC) based on the initial position of the active contour and the direction of cell movement. Using actual intravital experiments, we compare the performance of the proposed active contour tracker with the Dirichlet BC, the active contour tracker without the BC, the correlation tracker and the centroid tracker. The comparative results provide evidence of the advantages of the proposed method in terms of increased number of frames successfully tracked and reduced localization error.