Genetic changes related to carcinogenesis are being studied using hybrids of human lung carcinoma cells with normal human bronchial epithelial cells. Initial studies suggest that a limited population doubling potential (mortality) is the dominant genetic trait in hybrids cells. Other hybrid cell lines have been isolated and are being characterized for doubling potential, karyotype and tumorigenicity in athymic nude mice. The effects of individual chromosomes are being assessed by fusion with mini-cells containing single marked chromosomes.