T cells bearing the class II-restricted, DO-T cell receptor (TCR) are CD4+ if their thymocyte precursors are positively selected on the class II protein, IAd, but they are almost all CD4- after positive selection on a class II for which they have higher avidity, IAb. DO-TCR+ T cells mature in H-2b mice lacking CD4. CD4- DO-TCR+ T cells appear in H-2b mice at the same rate as their CD4+ counterparts appear in H-2d animals, suggesting that the CD4- cells are not the product of some minor pathway of thymocyte development and selection. In H-2b CD4 knock out mice expressing human CD2 under the control of the mouse CD4 promoter, mature DO-TCR+ cells did not express human CD2. These results suggest that the CD4-CD8-, DO-TCR+ mature T cells have developed without ever passing through the equivalent of a CD4+,CD8+ stage. The early expression of alpha/beta receptors (TCRs) on thymocytes in TCR transgenic mice may allow maturation of this type. Passage through the equivalent of the CD4+ CD8+, double-positive stage is not essential for differentiation of thymocytes into mature T cells.