Genetic mosaic mice can be produced by aggregating, during cleavage stages, the blastomeres of two embryos of different genotype into a single cluster, and by transferring the developing aggregates to the uterus of a surrogate mother. Substantial numbers of such composite embryos survive past birth. Among the living adult mosaic mice are individuals within which cells of markedly different immunogenetic constitution coexist. Through the incorporation of appropriate genetic markers into mosaics, many new possibilities now present themselves for analysis of biological problems during embryonic as well as adult life.