Characterization of surface molecules in relationship to human B lymphocyte differentiation will be continued in this study. The disappearance of surface Ig, Ia, and complement receptors will be used to mark B cell differentiation to mature plasma cells. Factors which influence B cell proliferation and maturation will be studied. Mitogens which stimulate B cells to develop only to intermediate stages will be utilized to determine if some of the T cell factors are specific for the terminal differentiation of B cells. Attempts will be made to isolate and characterize T cell factors. Detailed analysis of surface marker expression in relationship to the cell cycle will be carried out in vitro with lymphoblastoid cell lines. Special emphasis will be placed on the identification of the differentiation block of leukemic B cells and circumvention of this block so that terminal differentiation will be permitted. The further understanding of B cell differentiation will have implications in other cellular interacting systems and may allow new approaches to the management of human disease.