Scid mice lack functional lymphocytes because they carry a mutation that impairs rearrangement of immunoglobulin and T-cell receptor (TCR) genes. Rearrangement of TCR delta, but not gamma and beta genes, was routinely observed in DNA of scid thymocytes and thymocyte hybridomas. TCR delta gene rearrangements appeared to involve D delta 1, D delta 2, and J delta 1 elements only; rearrangement of elements upstream of D delta 1 (e.g., V delta 1) was not observed, and transcripts corresponding to fully assembled TCR delta genes (VDJ delta or VDDJ delta) were not detected in RNA from scid thymocytes. These findings suggest that D delta 1, D delta 2, and J delta 1 may be among the first TCR gene elements to undergo recombination and that scid T-lineage cells are developmentally arrested during or shortly after this stage of differentiation. One class of TCR delta recombination fragments (D delta 2-J delta 1) was amplified by the polymerase chain reaction (PCR) and cloned, and the recombination junctions were sequenced. Most fragments showed normal coding joints. Interestingly, five of seven coding joints that lacked N insertions showed evidence of recombination between short stretches (2-3 bp) of homologous sequence. As discussed, the general absence of V delta-, J gamma-, and J beta-associated rearrangements, despite the occurrence of normal D delta 2-J delta 1 rearrangements, raises the possibility that the scid mutation may cause premature cessation of TCR gene recombination and thereby arrest early T-cell development.