This chapter presents examples of how B cell tumors have been used to provide information about the various stages of the B cell differentiation pathway. It deals with studies of how these tumors may be used to investigate the regulatory mechanisms which trigger progression from one differentiation stage to another. The differentiation of cells within the B cell lineage along the pathway toward antibody secretion has a number of recognizable steps. These steps include both activation and differentiation events. The isolation of cells at any one of the distinct stages of B cell differentiation is difficult. One approach to the analysis of B lymphocyte ontogeny and differentiation is to use immortal, clonal cell lines including lymphomas and plasmacytomas. A particularly useful application of malignant B cells is the identification and characterization of cell membrane antigens associated with B cell development and the definition of cell subsets within the differentiation pathway.