The existence of extranuclear hereditary factors implies the presence of an extranuclear nucleic acid, to a certain degree different from nuclear DNA.' The presence of DNA in mitochondria, plastids, and kinetoplasts with melting profiles, reannealing properties, and buoyant densities different from those of the corresponding nuclear DNA's has by now been demonstrated by various groups (e.g., see refs. 1-7). There are only a few reports regarding hybridization reactions between DNA from extranuclear sites with the corresponding nuclear DNA. Shipp et al.,5 using tobaccochloroplast DNA and nuclear DNA of matured tobacco leaves, found 10-15 per cent binding of P-labeled nuclear DNA fragments and 0.6 per cent binding of P-labeled chloroplast DNA fragments to nuclear DNA embedded in agar, indicating no detectable homology. Dawid9 reported that frog-egg DNA fragments, apparently of mitochondrial origin,10 (at the concentrations used) do not compete with C14-labeled tadpole DNA in their reaction with homologous single-stranded frog DNA embedded in agar. He further showed in hybridization experiments that egg complementary RNA hybridized to a small degree with erythrocyte or liver nuclear DNA. The present paper will present data on hybridization reactions between nuclear and kinetoplast DNA's of Leishmania enriettii, and nuclear and mitochondrial DNA's from mouse liver. Materials and Methods.-Preparation of Leishmania DNA's: Kinetoplast and nuclear DNA of Leishmania were prepared by methods described previously.7 Each batch of kinetoplast DNA was characterized by the formation of only one rapidly appearing band following density gradient centrifugation in CsCl (d = 1.699, 44,770 rpm, 18 hr).11 C'4-labeled Leishmania nuclear DNA was obtained by the addition of C14-thymidine (100 yAe in 0.8 mg) to two Erlenmeyer flasks each containing 40 ml Senekjie medium, and inoculated with ca. 1.2 X 107 organisms each. The organisms were harvested after 3 days, filtered through cheesecloth, and centrifuged, and the pellet was washed once with water. The DNA was then prepared as previously described,7 and fragmented by mechanical shear of 7.0-10.5 X 106 gm/cm2 to a molecular weight of 3-5 X 101 (ref. 12). Preparation of mouse-liver DNA's: Mouse mitochondrial DNA was prepared as described before,'1 with some modifications. The DNase treatment was eliminated. The final pellets were further purified by washing with saline, and centrifuged at 18,000 g for 20 min, and the bottom layer was discarded. After addition of the solution containing Tris, sodium lauryl sulfate, and pronase, as described before. the pellets were digested. The preparation was twice extracted with phenol, each time followed by centrifugation in a Spinco ultracentrifuge in a SW25 rotor at 16,000 rpm for 20 min.3 The bottom layer and the interphase were discarded. Further purification proceeded as described before, except that the preparation was sheared at 7.0-10.5 X 106 gm/cm2 before it was placed on the Sephadex column. Each batch of mitochondrial DNA was characterized ...