A cloned rabbit DNA fragment containing the adult beta-globin gene has been introduced into the mouse germ-line by DNA microinjection into the pronuclei of fertilized mouse eggs. Twenty-four adult mice derived from eggs injected with the cloned DNA were screened for the presence of the rabbit gene in their liver DNA by Southern blot hybridization. Nine of these mice contain the rabbit gene in liver and at least six of them transmit it through the germ-line to a fraction of their offspring. Thus, it should be possible to derive permanent strains of mice carrying the foreign gene. The estimated numbers of copies of the rabbit gene per diploid mouse genome range from one or two up to 20 or more. The multiple copies of the rabbit DNA clone appear to be arranged in tandem head-to-tail arrays that are integrated into mouse chromosomal DNA. Chromosomal integration has been directly demonstrated for one mouse by in situ hybridization to metaphase chromosomes using a radioiodinated probe. In this mouse, most if not all the copies of the rabbit DNA fragment are integrated into one homologue of mouse chromosome 1. The possible use of this technique for the study of globin gene regulation is discussed.