T precursors from fetal liver and adult bone marrow were compared for their ability to give rise to V gamma 4+ T cell development. Fetal thymic lobes were repopulated with fetal liver or adult bone marrow cells, and the organ-cultured thymocytes were analyzed for their T cell receptor expression by the polymerase chain reaction (PCR). Both day 14 fetal liver and adult bone marrow cells gave rise to thymocytes with V gamma 4-J gamma 1 transcripts. However, the average size of the PCR products derived from adult precursors was slightly larger than that from fetal precursors. DNA sequence analysis of the V gamma 4-J gamma 1 transcripts showed that early fetal liver precursors predominantly gave rise to thymocytes with the V gamma 4-J gamma 1 transcripts without N nucleotide insertion, while late fetal liver and adult marrow precursors predominantly gave rise to thymocytes with modified V gamma 4-J gamma 1 junctions. These results suggest the possibility that the level of the N nucleotide insertion is programmed at the level of thymic precursors. This study also supported the model presented previously that the developmental potential of hematopoietic stem cells may change during ontogeny.