Cell fates were traced by injecting horseradish peroxidase into single blastomeres of Xenopus embryos at 2- to 512-cell stages. At later stages the number, types and locations of all labelled progeny were observed. Progeny of a single labelled ancestral cell divided coherently until the 12th cell generation, the onset of gastrulation, and then dispersed and mingled with unlabelled cells. Cell mingling was restricted at mediolateral and anterior-posterior boundaries. These boundaries were always respected by progeny of any blastomere labelled at the 512-cell stage but they were frequently crossed by progeny of blastomeres labelled at the 256-cell or earlier stages. The boundaries defined seven morphological compartments each populated exclusively by a group of ancestral cells at the 512-cell stage. Each blastomere that contributed progeny to the nervous system also gave rise to a wide range of cell types in all three primary germ cell layers but the clone was restricted to a single compartment. Analysis of clonal restriction of cell mingling was done in vitro. Twenty to thirty blastomeres were excised from one ancestral cell group at the 512-cell stage and combined in vitro with 20-30 blastomeres from another group. One group of blastomeres labelled with horseradish peroxidase was placed in contact with another group of unlabelled blastomeres, maintained in vitro for up to 2 days, and then processed histologically to show the distribution of labelled and unlabelled cells. Mingling was significantly greater in combinations of two of the same ancestral cell groups than in combinations of two different ancestral cell groups. A similar result was observed when a single labelled cell was combined with either the same or different ancestral cells. In all experiments the cells were significantly larger in combinations of different ancestral cell groups, indicating that they had undergone fewer divisions. These results are consistent with the hypothesis that boundaries observed in vivo are lines of clonal restriction formed by mutual inhibition of cell motility and cell division following contact between progeny of different ancestral cell groups.