Hearts of newborn mice were cut into small pieces, the fragments transplanted under the ear skin of adult recipients, and the graft survival followed visually (pulsating fragments were considered viable). Donor-recipient combinations were chosen from H-2 congenic (recombinant and mutant) strains in such a way as to provide differences in the entire H-2 complex or in only a small portion of it. The data obtained indicate that a difference between the donor and the recipient in either K, D, or I regions suffices for the rejection of the heart fragments. The rejection is often accompanied by the production of antibodies against classical H-2 antigens (in the case of K- or D-region disparities) or Ia antigens (in the case of I region disparities). In some instances, the antibodies persist in the recipient for more than 50 days. We conclude from these experiments that the same loci that cause acute skin graft rejection (H-2K, H-2D, and H-2I) are responsible for heart graft rejection. Furthermore, we also conclude that serologically Ia-negative tissues may carry Ia antigens in sufficient quantities to stimulate the production of Ia antibodies.