This study showed how immunomagnetic monodisperse microspheres (Dynabeads) can be used for rapid selection of stable and homogeneous sublines of cancer cells. The beads were activated with antibody to a specific surface membrane marker. We used a monoclonal antibody (MAb) to human secretory component (SC), the epithelial receptor for polymeric immunoglobulins (pIg). A relatively low number of particles favored rosetting of strongly positive cells. SC-expressing cells were isolated within 30 min from a colonic carcinoma cell line (HT-29.E10). After 13 months of continuous culture the selected cells had a 15 times higher cellular content and a 10 times higher secretory rate of SC than parallel cultures of the original cell line. Immunomagnetic microspheres can be recommended in experimental cancer research, e.g., in order to minimize cellular heterogeneity and to isolate cells with particular surface membrane phenotypes in differentiation studies. The selection method is rapid and simple, and eliminates laborious test systems for conventional subclone screening.