A characteristic abnormality of chimeras composed of wildtype and T/T (Brachyury) mutant embryonic stem cells is the aggregation and accumulation of mutant cells in the primitive streak and its descendant, the tail bud (V. Wilson, L. Manson, W. C. Skarnes, and R. S. P. Beddington (1995). Development 121, 877-886). To demonstrate that this aberrant behaviour of mutant cells in the streak is due only to the absence of wild-type T protein and to investigate dosage effects of T function on cell deployment during gastrulation, a vector expressing T under the control of its own promoter (which results in T expression in the primitive streak but not in the notochord) was introduced into T/T mutant ES cells carrying a ubiquitous lacZ lineage marker. Four clones (TR clones) that express T appropriately in the streak and rescue abnormal chimeric morphology were recovered. In chimeras, these four clones fall into two distinct categories with respect to their ability to exit from the primitive streak and their subsequent tissue colonisation profile. TR1 and TR4 descendants no longer accumulated in the tail bud and gave rise to all types of mesoderm as well as colonising ventral neurectoderm. Interestingly, TR2 and TR5 cells (which express higher levels of T protein than TR1 and TR4 in vitro) tended to exit the streak prematurely, showed a marked reduction in posterior mesoderm colonisation, and were virtually excluded from ventral neurectoderm. However, while descendants of all four TR clones can colonise dermomyotome at all axial levels, the parent T/T mutant cells only contribute to this tissue rostral to the forelimb bud and are completely excluded from more caudal dermomyotome. These results show that the abnormal aggregation of mutant cells homozygous for the Brachyury deletion (approximately 200 kb) can be ascribed solely to the lack of wild-type T protein, as can the failure of T/T cells to colonise caudal dermomyotome. They also suggest that patterns of cell recruitment from the streak can be influenced by the level of T expression.