Preimplantation mouse embryos were infected with a recombinant retrovirus, which serves as a genetic marker for the progeny of an infected blastomere. Quantitative analysis of proviruses carried in mosaic animals indicated that the molarity of individual proviral bands was equal in almost all tissues. The cells that give rise to the embryo proper must therefore intermingle extensively before final tissue allocation to ensure equal contribution of founder cells to all somatic tissues. The distribution of molarities of individual proviruses suggested that somatic lineages are derived from at most eight founder cells. About half of the proviruses were present in the germ line and the somatic tissues of mosaic animals, while the remaining proviruses were found either in the germ line or in the somatic tissues, but not in both. Our results suggest that at least three cells form the germ line and are set aside prior to somatic tissue allocation.