Introduction of a transgenic alpha beta TCR (V alpha 2, V beta 8.1) specific for lymphocytic choriomeningitis virus (LCMV), in the context of H-2Db 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 alpha beta transgenic mice, in particular the earliest CD4-8- stages. 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-CD44- expresses the transgenic TCR, and the earlier CD25-CD44+ and CD25+CD44- do not. Furthermore, in contrast to other TCR alpha beta transgenic lines, TCR gamma delta lineage cells appear to develop normally.