In this report, we have characterized T cell ontogeny in mice transgenic for the V gamma 1.1J gamma 4C gamma 4 (TcR gamma 4) TCR chain gene by analyzing the T cell surface phenotype and microarchitecture of the transgenic lymphoid organs during development. The first wave of V gamma 3J gamma 1C gamma 1 TCR+ thymocytes that appear in the thymus at day 14 of gestation were virtually absent in the TCR gamma 4 transgenic mice. However, comparable levels of total gamma delta+ thymocytes were present in the TCR gamma 4 transgenic mice, suggesting that these thymocytes expressed a transgene receptor. In addition, the appearance of significant numbers of TCR alpha beta+ T cells occurred earlier (day 17 of fetal development) in the transgenic thymus. After birth, thymuses from TCR gamma 4 transgenic mice were characterized by a consistent increase in the number of TCR gamma delta+ thymocytes and a transient increase (between 2 and 4 wk of age) in the absolute number of TCR+ thymocytes (2- to 4-fold) compared with thymuses from normal mice. Immunohistologic analysis of the thymus, spleen, and lymph nodes from 2-day and 3-wk-old transgenic mice showed significant differences to controls only in regions harboring mature, functional T cell subsets which may be the result of bidirectional signaling between lymphocyte subsets (influenced by transgene expression) and stromal elements. The results demonstrate that striking differences, leading to the earlier appearance of mature T cells, occurred in both the fetal and neonatal thymus and periphery of mice that expressed a TCR gamma 4 transgene. These findings are consistent with the hypothesis that expression of the TCR gamma 4 chain gene, early in ontogeny, has a positive influence on the development of the T cell compartment.