ABSTRACT-In an attempt to separate malignant from normal and reactive stromal cells, we fractionated ascites cells from BALB/c mice bearing a transplantable myeloma (MPC-11) by isopyknic centrifuga-tion in continuous density gradients of povidone-coated silica gels (Percoll~, Cells from different fractions were then analyzed by mor-phologic and immunologic criteria. The ability of cells from the different fractions to form colonies in soft agar and to produce tumors in BALB/c mice was also examined. Although most fractions contained morphologically identifiable plasma cells, colony-forming cells (CFC), derived from multiply passaged tumors, separated in a sharp peak at 1.072 g/ml. CFC peaked at 1.078-1.082 g/ml for tumors passed less than three times and were invariably markedly depleted from the low-density portions of the gradients. Cells recovered from different frac-tions of the gradients were cultured in soft agar and inoculated sc into syngeneic mice. In these experiments, a highly significant correlatio
