Leukemogenesis is a complex process involving an accumulation of genetic lesions affecting both growth and differentiation in cells of the hematopoietic lineage. Our laboratory has established a non-producer v-myb-transformed cell line (BM2/C3A) which, when injected into the chicken embryo, does not produce leukemia. Recently, a spontaneous variant of this cell line, called BM2L, was obtained from in vivo experiments. BM2L produces an acute monoblastic leukemia when injected into the chicken embryo. BM2L cells do not differentiate in vivo or in vitro, but continue to proliferate under conditions in culture that allow for the differentiation of BM2/C3A cells into macrophages. In addition, BM2L cells have reduced requirements for exogenous growth factors. BM2L cells contain the v-myb allele and express v-Myb protein, but leukemogenicity does not involve point mutations in v-myb. The BM2 model, consisting of two non-producer cell lines differing in vivo in their leukemogenicity, provides a novel system for identifying genes that play a role in the induction or suppression of leukemogenesis.