The T cell alloantigen Tthyd appears on a subpopulation of thymocytes in mice bearing the Igh-1d or e heavy chain haplotype. Ontogenetic studies have suggested that this antigen precedes the appearance of two other T cell alloantigens in the same linkage group, Tindd and Tsud. The purpose of this study was to determine if the Tthy-bearing cell is a precursor for cells in the periphery expressing Tind and Tsu. CAL-20 mice were treated at 48-h intervals beginning on the day of birth with a monoclonal antibody recognizing Tthy. Tthy alloantigen expression, monitored by cytotoxicity assays, was found to be significantly depressed in the thymuses of treated animals; Tind and Tsu also failed to appear in the periphery. Treatment with anti-Tthy caused no significant changes in frequency or surface intensity in the expression of surface Ig. Thy-1.2, Thy-1.2, and Lyt-2.2, as studied by cytofluorograph analysis. We conclude that the T-thyd-bearing cells in the thymus represent a subpopulation that may be a precursor for Tindd- and Tsud-bearing cells. However, Tthyd-bearing cells are more mature than the Thy-1.2 common T cell precursor, pre-T.