We have characterized transgenic mice carrying a functional T cell receptor (TCR) Cγ4 (Vγ1.1Jγ4Cγ4) gene. Results indicate that active transcription of the Cγ4 transgene can influence expression of the endogenous Cγ4, Cγ1 (Vγ3-, Vγ4-, Vγ2-, or Vγ5Jγ1Cγ1) and Cγ2 (Vγ1.2Jγ2Cγ2) genes, while the ultimate expression of other TCR δ, α, and β chain genes, as well as the adult T cell response, are relatively unaltered. Cells expressing transgenic Cγ4 and endogenous δ TCR transcripts can migrate to the skin as dendritic epithelial cells (DEC) even though Cγ4 cells are rarely, if at all, found in the skin. Transgenic and control mice were compared at 2 weeks, 6-7 weeks, and older. At 2 weeks, the thymus of transgenic mice, particularly the medulla, was much larger than control. Moreover, peripheral lymphoid tissues of younger mice were markedly (as much as 100-fold) more immunoreactive (both Con A response and alloreactivity). These differences, although persistent, became smaller in older mice. The data suggest that transgene expression has a major effect on T cell development and reactivity.