Cloned cell populations derived from neuroblastoma tumor, when grown in normal tissue culture medium containing 10-20 percent serum, divide at a geometric rate and are quite undifferentiated morphologically. However, if the serum is removed from the medium, cell division is arrested, and within several hours a high degree of cell differentiation is attained. In the present application it is proposed to use this system as a model of benign and malignant phases in the same cell population. This model is of great interest since the benign phase is induced by removal of one of the natural components of the cellular environment rather than through addition of an extraneous inhibitor. Moreover, transformation from malignant to benign in this case is not likely to be associated with a genetic change; rather, the environmental change probably induces modification of the phenotype expression. Thus, the present model may yield valuable information on the therapeutic control of malignant tumors. The cloned nature of this cell population makes it particularly suitable for a comparative study of malignancy. The research proposed here would attempt to clarify some of the mechanisms underlying transformation from the malignant to benign phase. Attention will be directed to cell membrane structure and function in the two phases. Biochemical analysis of lipid composition of the membrane will be carried out in the benign and malignant phases in the same population. Changes in fixed charge density will be evaluated in cell electrophoresis, and membrane function in the two phases will be examined through use of electrophysiological techniques involving intracellular recording from single cells combined with ionic assays in the bulk.