Rapid and efficient cell sorting methods are important for tissue progenitor cell isolation. We built and evaluated a laboratory prototype of a continuous flow, quadrupole magnetic cell sorter. The sorter was tested on a model cell system of human peripheral lymphocytes. The helper T cell subpopulation was targeted by primary, mouse anti-CD4 monoclonal antibody conjugated to a fluorochrome (FITC), and magnetized by secondary, anti-FITC antibody magnetic colloid. The purities and recoveries of the cell fractions were measured by flow cytometry and an automated cell counter. Cells were spread across the flow according to their magnetophoretic mobilities. The purity of the CD4 cell enriched fraction was 99.6%, and the purity of the CD4 cell depleted fraction was 2% for an initial CD4 cell purity of 36%; the corresponding recovery of the enriched CD4 cell fraction was 59% at a sorting speed of 4,200 cells/s (four experiments). The recovery could be increased to 90% with a concomitant decrease in the purity of CD4 cell enriched fraction to 66%. This type of sorting should be applicable to any cells in suspension for which a suitable antibody exists, in particular, to large, fragile cells. ASAIO Journal 1999; 45:127–130.