Transgenic mice bearing a c-myc oncogene under control of the immunoglobulin heavy chain enhancer (Eµ-myc mice) reproducibly develop and die from tumors of the B lymphocyte lineage (J. M. Adams, A. W. Harris, C. A. Pinkert, L. M. Corcoran, W. S. Alexander, S. Cory, R. D. Palmiter, and R. L. Brinster, Nature (Lond.), 318: 533–538, 1985; W. Y. Langdon, A. W. Harris, S. Cory, and J. M. Adams, Cell, 47: 11–18, 1986; A. W. Harris, C. A. Pinkert, M. Crawford, W. Y. Langdon, R. L. Brinster, and J. M. Adams, J. Exp. Med., 167: 353–371, 1988; reviewed in S. Cory and J. M. Adams, Annu. Rev. Immunol., 6: 25–48, 1988). Analysis of lymphocytes obtained by serial sampling of peripheral blood from individual hemizygous (Eµ-myc/0) and homozygous (Eµ-myc/Eµ-myc) transgenic mice indicates that proliferation in the original host and transplantability into histocompatible recipients are distinct properties that can be acquired independently and in either order. These two types of transgenic mice differ in that homozygous mice have about one-fourth the life span of hemizygous mice and develop polyclonal, non-transplantable tumors in comparison to the oligoclonal, highly transplantable malignancies seen in hemizygous animals. In conclusion, the overall concept of malignancy is best viewed as an aggregate of the separable parameters of cellular proliferation, clonality, tissue invasiveness, metastasis, and (experimental) transplantability. The Eµ-myc transgenic mouse represents an attractive model in which to investigate the multistep nature and alternative pathways of tumorigenesis.