While a great deal has been learned about the genetic control of B lymphocyte behavior through the observation of primary B cells from mice bearing targeted gene mutations, such studies can be restricted by the limited number and longevity of the cells ex vivo, by their heterogeneity, and by the inability to apply further genetic manipulations. Here we describe a protocol for the efficient derivation of mutant B lymphoma cell lines, by crossing mice bearing targeted mutations in genes affecting mature B cell function with transgenic mice bearing the lymphomagenic Emu-myc transgene. Pre-B and B lymphomas were obtained with high frequency, and the cells were readily adaptable to culture. The B lymphoma lines bore surface markers consistent with an immature phenotype and were amenable to cloning and to stable transfection. They are currently being used as a well-defined and unlimited cell source to study antigen receptor signalling and B cell-specific gene regulation, and the dependence of these processes on the products of the vav, CD45, lyn, oct-2, btk and OBF-1 genes.