We have shown that beginning with 14-day mouse embryo thymus, rearrangements of T cell receptor beta-chain genes and transcripts of alpha and beta chains culminating in the generation of T cell receptor-bearing cells can be obtained in organ cultures. We have failed to detect rearranged beta chain genes, and beta chain transcripts or membrane receptors in concanavalin A-activated nude spleen cells, although we cannot exclude the possibility that a minor subpopulation of nude Thy-1+ cells may express T cell receptors. Our results suggest that the thymus is both necessary and sufficient for generation of T cell receptor-bearing cells and that organ culture provides a system for further investigation of the mechanisms involved.