High gradient magnetic retention of paramagnetic red blood cells was measured under conditions which gave rise to only partial capture of the cells in each sample. The dependence on buffer flow rate, magnetic field, and particle paramagnetic susceptibility was determined and compared with theoretical models. In addition, parallel and transverse geometries (field parallel to, or perpendicular to, flow) were compared and no difference was observed in the probability of capture of weakly paramagnetic cells in a lightly loaded filter bed.