Introduction of a transgenic c// TCR (Vo2, V]/8.1) specific for lymphocytic choriomeningitis virus (LCMV), in the context of H-2D into the genome of C57BL/6 mice, has many effects on the development and selection of T cells in both the thymus and the periphery. These mice produce increased numbers of CD4-8 mature T cells, all of which express the transgenic TCR, and small numbers of CD4/8 cells using endogenous TCRs are also produced. This study follows the intrathymic development of T cells in these TCR o]/transgenic mice, in particular the earliest CD4-8stages. As expected, the transgenic TCR is expressed on the cell surface at an earlier developmental stage than endogenous TCRs in nontransgenic littermate controls. Of the three major subsets expressing the heat-stable antigen (HSA), only the most mature, the CD25-CD44expresses the transgenic TCR, and the earlier CD25-CD44 and CD25/CD44 do not. Furthermore, in contrast to other TCR o]/transgenic lines, TCR lineage cells appear to develop normally.