Cloning of cells from peri-implantation embryos by blastocyst injection was used to investigate the time of X-chromosome inactivation in that part of the ectoderm lineage giving rise to foetal tissues of the mouse. Matings were arranged so that the two X-chromosomes of female donor cells controlled two distinct coat colours and host blastocysts were of a third colour genotype. No coat chimaeras were obtained in experiments using donor cells from the primitive ectoderm of 6th or 7th day embryos or from lactationally delayed implanting or reactivated blastocysts. In contrast, a minimum of 80 unequivocal coat chimaeras were obtained in experiments in which primitive ectoderm cells from 5th day implanting blastocysts were used for injection. The majority of these chimaeras that had received a female cell exhibited both donor colours in addition to host colour in their coats, suggesting that the donor cell had not undergone X-inactivation until one or more cycles after transplantation. The remainder of such chimaeras exhibited only one or other donor coat colour. Determination of the parental origin of the allocyclic X-chromosome in donor metaphase preparations in internal tissues of several chimaeras revealed that the coat pattern did not always reflect the X-activity status of the donor cell clone as a whole. Nevertheless, the findings suggest that X-inactivation takes place shortly after implantation in the primitive ectoderm cell population from which the foetus is derived. Of the 68 chimaeras in which the sex of both the donor and host component was established 62 proved to be fertile. Furthermore, 21 of the 37 fertile chimaeras whose sex corresponded with that of the donor cell yielded functional gametes of donor origin. Injection of cells from a single donor blastocyst into a series of host blastocysts established that at least 2 cells in 5th day primitive ectoderm can give rise to both somatic cells and functional germ cells among their mitotic descendants.