Normal human foreskin fibroblasts treated in vitro with a chemical carcinogen or irradiated with ultraviolet light subsequently acquired anchorage independent growth and an extended but finite capacity for exponential growth. All cell lines were derived from cells recovered from colonies that had grown in semisolid medium; cell lines originally treated with a chemical carcinogen produced nodules after s.c. inoculation into nude mice. G-banding analysis of 10 cell lines (including one ultraviolet light line) revealed that seven were chromosomally abnormal with structural and numerical chromosome alterations, one was characterized by a consistent trisomy, and the other two were normal diploid. Structural alterations consisted of chromosome deletions, translocations, and partial chromosome duplications. Although no common structural or numerical abnormality was detected, several structural alterations were observed involving chromosomes 1, 7, 11, and 22, where fgr, erb-B, H-ras-1, and sis protooncogenes, respectively, are located. In one cell line trisomy 17 was a unique chromosome alteration. The induction of chromosome changes may have influenced the proliferative capacity of the treated cells relative to nontreated cells. However, the two cell lines without detectable chromosome changes also had an increased proliferative life span, suggesting that other submicroscopic genetic alterations may have affected cell multiplication. Although carcinogen induced chromosome abnormalities may represent a step in the process of neoplastic development, additional genetic and/or epigenetic changes, are required for indefinite growth and the expression of malignancy.