Abelson murine leukemia virus (A-MuLV) directly transforms lymphoid cells in vitro in a quantitative fashion. Therefore, this virus probably causes leukemia by directly interfering with normal growth and differentiation of particular hematopoietic cells. The A-MuLV lymphoid transformation system will be used in this work as a base to further the understanding of virus-cell interaction in the neoplastic process. Two lines of investigation will be pursued: a. studies to determine what properties are required for transformation of lymphoid cells. b. studies to define whether the virus interacts only with specific target cells at a certain state of differentiation and to determine the basis for this interaction.