We have derived T cell lines from mice inoculated with Gross leukemia virus, which appear to represent early T cell developmental stages and to reflect normal T cell development. These cell lines may provide a breakthrough in the study of T cell development as Abelson transformants have done for the study of B cell development. Analysis of the TCR gene expression in these cell lines reveals that the sequence of rearrangement and expression of each TCR gene is not strictly ordered. Expression of RNA for the TCR alpha and -beta genes appears to be coordinated with rearrangement at the alpha and beta loci. This is not the case for gamma gene expression. Availability of the homogeneous populations of cells represented in these cells lines allows for a more detailed molecular analysis of T cell development than was previously possible.