Blood vessels originate as simple endothelial cell tubes. It has been proposed that platelet-derived growth factor B polypeptide (Pdgfb) secreted by these endothelial cells drives the formation of the surrounding muscular wall by recruiting nearby mesenchymal cells^1,2. However, targetted inactivation of the Pdgfb gene³ or the Pdgf receptor β(Pdgfrb) gene⁴, by homologous recombination, does not prevent the development of apparently normal large arteries and connective tissue. We have used an in vivo competition assay in which we prepared chimaeric blastocysts, composed of a mixture of wild-type (Pdgfrb^+/+) and Pdgfrb^+/− or wild-type and Pdgfrb^−/− cells, and quantified the relative success of cells of the two component genotypes in competing for representation in different cell lineages as the chimaeric embryos developed. This study revealed that the participation of Pdgfrb^−/− cells in all muscle lineages (smooth, cardiac, skeletal and pericyte) was reduced by eightfold compared with Pdgfrb^+/+ cells, and that participation of Pdgfrb^+/− cells was reduced by twofold (eightfold for pericytes). Pdgfrb inactivation did not affect cell contribution to non-muscle mesodermal lineages, including fibroblasts and endothelial cells. Chimaera competition is therefore a sensitive, quantitative method for determining developmental roles of specific genes, even when those roles are not apparent from analysis of purebred mutants; most likely because they are masked by homeostatic mechanisms.