Bronchial epithelial cells were cultured from an individual with no evidence of malignant disease. These cells, designated HB56B, had a greatly extended in vitro life-span, being able to undergo 50 passages and 200 population doublings in contrast to the usual 3 to 4 passages and 20 to 30 population doublings characteristic of normal human bronchial epithelial cells. HB56B cells had karyotypic evidence of an amplified region on the short arm of chromosome II. Unlike normal bronchial epithelial cells, which undergo terminal squamous differentiation in vitro in response to fetal bovine serum, HB56B cells were only minimally affected by serum. These cells were readily established as an immortalized cell line, HB56B/5T, following transfection with a plasmid containing SV40 early region DNA. HB56B cells were non-tumorigenic in athymic nude mice, but HB56B/5T cells within a few passages of transfection with the SV40 plasmid formed tumors of which 28/37 regressed. HB56B cells may offer an experimental system for the study of proliferation, differentiation, and senescence control in human bronchial epithelial cells.