The production of chimaeric mice by aggregating pre-implantation mouse embryos or by injection of cells into the blastocyst has been of great value in analysing the regulation of early mammalian development and in dissecting the relationships of early cell lineages. While the totipotent cells of the pre-implantation embryo can be grown in vitro and thus are readily accessible to experimental manipulation, this is not possible after the embryo has implanted into the uterus. This problem has severely hampered the analysis of cell migration and of cell lineage relationships in later stages of mammalian development. In contrast, the chicken embryo can be manipulated experimentally throughout embryo-genesis and this has made the bird a favourable system for studying patterns of cell migration in the development of higher vertebrates. In mammals, the introduction of retroviruses and haematopoietic cells has provided two means of probing post-implantation development by direct intervention. I report here that cultured neural crest cells, when microinjected into 9-day-old mouse embryos, can migrate over considerable distances and participate in normal development, and the resulting chimaeric animals show pigmentation derived from the donor cells in hair and iris. The introduction of cells into post-implantation embryos may provide the means of studying patterns of cell migration in mammalian development at a level of sophistication which so far has been restricted to the chicken system.