A mouse strain with low lung tumor susceptibility (C3H) and a strain with high lung tumor susceptibilit (A/J) were reciprocally crossed to produce C3A and AC3 F1 hybrid mice. Ki-ras oncogenes were detected in spontaneous and chemically induced lung tumors obtained from the C3Aand AC3 mice. To further explore the genetics of the Ki-ras gene in mouse lung tumor susceptibility, the parental origin of Ki-ras oncogenes detected in lung tumors from the F1 hybrids was determined by a strategy based on a 37-base-pair deletion in the second intron of the A/J Ki-ras allele. (ii) Spontaneously occurring and chemically induced lung tumors from the A/J mouse were found to have a high frequency (88-100%o) of activated Ki-ras protooncogenes (7, 8). Ki-ras oncogenes have also been detected in spontaneous and chemically induced lung tumors from C3H mice; the tumors induced in C3H mice after repeated carcinogen administration, however, were smaller in size at the time of sacrifice, the tumor multiplicity was lower, and the latency period for tumor development was much longer than in the sensitive A/J mice (9). These data suggest that spontaneous and chemically induced lung tumors from the C3H x A/J (C3A) and A/J x C3H (AC3) F1 hybrids should contain Ki-ras oncogenes. To further understand the genetics of the Ki-ras gene in mouse lung tumor susceptibility, we examined the parental origin of the activated Ki-ras protooncogenes detected in lung tumors from C3A and AC3 hybrid mice.MATERIALS AND METHODS Lung Tumor Induction. The C3H/HeJ and A/J parents of the C3A and AC3 F1 hybrids originated from mice purchased from The Jackson Laboratory. Spontaneous lung tumors were collected from untreated 2-year-old C3A mice. Six-to 8-week-old C3A or AC3 mice were given i.p. injections of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 50 mg/kg), dissolved in phosphate-buffered saline, three times per week for 8 weeks. Controls were given i.p. injections of saline solution. Vinyl carbamate (VC) was administered to 7-week-old C3A or AC3 mice in a single injection (either 60 or 20 mg/kg). Tumors were harvested from chemically treated animals between 6 and 14 months of age. This time frame oftumor collection enabled us to obtain both adenomas and adenocarcinomas. Lung tumors were collected, frozen in liquid nitrogen, and then stored at -80°C. Before freezing, a representative portion of each tumor was fixed in neutral buffered 10%o formalin for histopathological examination.Polymerase Chain Reaction (PCR). The primer-directed enzymatic amplification of specific Ki-ras DNA sequences was carried out as described (10) with Taq DNA polymerase. Reactions were carried out in a 100-,l mixture that contained 2 ,ug of DNA, 50 mM KCl, 10 mM Tris (pH 8.4), 2.5 mM MgCl2, 200 ,uM dNTPs, gelatin at 200 ,ug/ml, 2 units of Taq DNA polymerase (Perkin-Elmer/Cetus), and two primers. Samples were subjected to 25 cycles of amplification in a thermal cycler (DNA denaturation at 94°C for 1 min, primer annealing at 45°C for 2 min, and extension at 72°C for 2 ...