Transformation of lymphoid and fibroblastic cells by Abelson murine leukemia virus (A-MuLV) is mediated by the viral tyrosine protein kinase. We do not yet know the important target proteins in the cell, the host proteins that modulate the kinase activity, or the host proteins involved in the signal-transduction pathway ultimately leading to altered patterns of cell growth. As a first step toward identifying these host proteins, we have isolated and characterized several flat revertant cell lines from transformed lines carrying v-abl. Clonal transformed cell lines used as parental strains were prepared by infecting Rat-2 fibroblasts with A-MuLV, using M-MuLV as helper. A rhodamine dye screening procedure was used to obtain three clones of morphologically flat revertant cells. Each of the three lines was non-refractile and contact inhibited. All the lines retained a transformation-competent copy of A-MuLV; all released high titers of virus capable of inducing foci on previously uninfected Rat-2 cells. Analyses of the revertant lines suggest that diverse mechanisms can lead to loss of transformed morphology.