THE zinc-finger transcription factor GATA-3 is expressed in haematopoietic cells and in the developing kidney and nervous system^1–7. Within the haematopoietic lineages, expression of GATA-3 is restricted to thymocytes and T cells. Functionally important GATA-3 binding sites have been identified in multiple T-cell-specific genes^1,6–8. Mice containing homozygous null mutations of the GATA-3 gene die on embryonic day 12, precluding a detailed assessment of the role of GATA-3 in haematopoietic development⁹. Here we have used murine embryonic stem (ES) cells containing homozygous mutations in the GATA-3 gene (GATA-3^−/− ) in conjunction with the RAG-2^−/− (ref. 10) and C57BL/6 complementation systems to study the role of GATA-3 in mammalian haematopoiesis. Our results show that GATA-3^−/− ES cells can contribute to the development of the mature ery-throid, myelomonocytic and B-cell lineages, but fail to give rise to thymocytes or mature peripheral T cells. The differentiation of GATA-3^−/−T cells is blocked at or before the earliest double-negative (CD4⁻/CD8⁻) stage of thymocyte development, such that the GATA-3^−/− ES cells are unable to contribute measurably to the double-negative thymocyte population. These findings suggest that GATA-3 is an essential and specific regulator of early thymocyte development.