This paper reviews the major reports of the spontaneous or carcinogen-induced transformation of human fibroblasts to the malignant state, to infinite lifespan, or to anchorage independence. In some cases, the transformed cells and the parent cell with which the work began were made available to us to be tested to determine whether the cells shared common isozymes, HLA antigens, restriction-fragment length polymorphisms, marker chromosomes, etc., as one would expect. When we examined the normal fibroblastic cell line KD for these markers, and the transformed HuT cell lines developed from it by Kakunaga (Proc. Natl. Acad. Sci. (U.S.A.), 75, 1334, 1978) for these markers, we found marked differences, indicating that KD cells and HuT cells are derived from different individuals. When we applied these techniques to the 3 human fibroblast cell lines transformed by Namba to acquire infinite lifespan in culture (Gann, 27, 221, 1981), it became clear that KSMT-6 was derived from the parent cell, KMS-6, but that both cell lines CT-1 and SUSM-1 were derived from the same parental cell line, AD387. Similar studies with other sets of cell lines are also reported. In the light of these studies, it appears that there is no example of the malignant transformation of human fibroblasts by carcinogen treatment. However, neoplastic transformation and transformation to infinite lifespan by carcinogen treatment have been achieved by a number of workers. We speculate as to how malignant transformation might be obtained by carcinogen treatment.